COMPOSITIONS AND METHODS FOR INCREASING CHOLESTEROL EFFLUX AND RAISING HDL USING ATP BINDING CASSETTE TRANSPORTER PROTEIN ABC1

Abstract: The present invention relates to novel ABC1 polypeptides and nucleic acid molecules encoding the same. The invention also relates to recombinant vectors, host cells, and compositions comprising ABC1 polynucleotides, as well as to methods for producing ABC1 polypeptides. The invention also relates to antibodies that bind specifically to ABC1 polypeptides. In addition, the invention relates to methods for increasing cholesterol efflux as well as to methods for increasing ABC1 expression and activity. The present invention further relates to methods for identifying compounds that modulate the expression of ABC1 and methods for detecting the comparative level of ABC1 polypeptides and polynucleotides in a mammalian subject. The present invention also provides kits and compositions suitable for screening compounds to determine the ABC1 expression modulating activity of the compound, as well as kits and compositions suitable to determine whether a compound modulates ABC1-dependent cholesterol efflux.

Description: COMPOSITIONS AND METHODS FOR INCREASING CHOLESTEROL EFFLUX AND RAISING HDL USING ATP BINDING CASSETTE TRANSPORTER PROTEIN ABCl This application claims priority to U.S. provisional application no. 60/140,264, filed on June 18, 1999: U.S. provisional application no. 60/153,872, filed on September 14, 1999; and U.S. provisional application no. 60/166,573, filed on November 19, 1999. TECHNICAL FIELD OF INVENTION The present invention relates to novel ABCl polypeptides and nucleic acid molecules encoding the same. The invention also relates to recombinant vectors, host cells, and compositions comprising ABCl polynucleotides, as well as to methods for producing ABCl polypeptides. The invention also relates to antibodies that bind specifically to ABCl polypeptides. In addition, the invention relates to methods for increasing cholesterol efflux as well as to methods for increasing ABCl expression and activity. The present invention further relates to methods for identifying compounds that modulate the expression of ABCl and methods for detecting the comparative level of ABCl polypeptides and polynucleotides in a mammalian subject. The present invention also provides kits and compositions suitable for screening compounds to determine the ABCl expression modulating activity of the compound, as well as kits and compositions suitable to determine whether a compound modulates ABC1- dependent cholesterol efflux. BACKGROUND OF THE INVENTION Circulating lipids in human plasma or lymphatic fluid consist of cholesterol, cholesteryl esters, triglycerides and phospholipids. These lipids are transported in large molecular complexes called lipoproteins, which consist of a core of cholesteryl esters and/or triglycerides, an envelope of phospholipids and free cholesterol, and apolipoproteins (Scriver et al., Eds., The Metabolic and Molecular Basis of Inherited Disease, 7 th Ed., p.1841-1851 (McGraw-Hill, New York 1995)). Apolipoproteins are involved in the assembly and secretion of the lipoprotein, as well as the activation of lipoprotein modifying enzymes, such as lecithin cholesterol acyl transferase (LCAT). In addition, apolipoproteins provide structural integrity and are ligands for a large spectrum of receptors and membrane docking proteins. The plasma lipoproteins are categorized into five types according to size: chylomicrons (largest in size and lowest in density), very low density lipoproteins (VLDL), intermediate density lipoproteins (IDL), low density lipoproteins (LDL) and high density lipoprotein (HDL). Chylomicrons, VLDLs. IDLs, and LDLs transport exogenous and endogenous cholesterol and triacylglycerols to peripheral sites, where the lipids play a role in various metabolic pathways and serve as a major constituent of cell membranes. Chylomicrons are assembled in the intestinal mucosa as a means to transport dietary cholesterol and triacylglycerols to various tissues. VLDLs are formed in the liver to transport endogenous cholesterol and triacylglycerols synthesized by the liver to extra-hepatic tissues, such as muscle and adipose tissue. In fasting serum, VLDLs contain 10-15% of the total serum cholesterol and most of the triglyceride. In circulation, VLDLs are converted to LDLs through the action of lipoprotein lipase. LDLs are the primary plasma carriers of cholesterol for delivery to all tissues, typically containing 60-70% of the total fasting serum cholesterol. In contrast, HDLs are involved in "reverse cholesterol transport", the pathway by which excess cholesterol is transported from peripheral sites back to the liver, where it is excreted in the form of bile salts (Glomset, J.A., J. LipidRes., 9, 155-167 (1968)). Nascent HDLs are synthesized de novo in the liver and small intestine, as protein-rich disc-shaped particles devoid of cholesterol and cholesterol esters. In fact, a major function of HDLs is to act as circulating stores of apolipoproteins, primarily apo C-I, apo C-II, and apoE. The nascent or protein-rich HDLs are converted into spherical lipoprotein particles through the accumulation of cholesteryl esters obtained from cellular sources. The HDL normally contain 20-30% of the total fasting serum cholesterol. According to current theories, the reverse efflux of cellular cholesterol to HDL is mediated through two mechanisms: an aqueous diffusion pathway and an apolipoprotein- mediated pathway. The relative importance of these distinguishable mechanisms depends on the cell type and metabolic state (Oram et al., J. LipidRes., 37:2743-2491 (1996); Rothblat et al., J. LipidRes., 40:781-796 (1999); Stein et al., Atherosclerosis, 144:285-301 (1999)). For many cells, the aqueous diffusion pathway is the principle pathway through which cholesterol efflux occurs (Johnson et al., Biochim. Biophys. Acta, 1085:273-298 (1991)). This pathway involves the bidirectional exchange of cholesterol between cell membranes and a lipoprotein acceptor, such as HDL, in the extracellular space through a process of passive transport (Remaley et al., Arterioscler. Thromb. Vase. Biol., 17: 1813- 1821 ( 1997); Rothblat et al., J. Lip. Res., 40:781 -796 (1999)). The exchange may occur primarily at surface microdomains known as caveolae (Fielding et al., Biochemistry, 34: 14288-14292 91995)). Net efflux can be driven by conversion of cholesterol in the exteracellular compartment to cholesteryl ester by the action of LCAT. Alternatively, in macrophage and fibroblast cells, cholesterol and phospholipid efflux is primarily mediated through apolipoproteins, such as apo A-I, apo A-II, and Apo E (Remaley, supra (1997); Francis, et al., J. Clin. Invest., 96:78-87 (1995); Vega et al., J. Intern. Med., 226:5- 15 (1989); Sakar et al, Biochim. Biophys. Acta, 1438: 85-98 (1999); Hara et al., J. Biol. Chem., 266:3080-3086 (1991); Fielding et al., J. Lipid Res., 38, 1503-1521 (1997); Oram et al., J. Lipid Res., 37, 2743-2491 (1996)). The process of apolipoprotein-mediated lipid efflux particularly dominates in macrophages and other scavenger cells when they are cholesterol-loaded and/or growth-arrested. Apolipoprotein-mediated efflux is an active transport process that requires the direct interaction of the apolipoprotein with the cell surface, the lipidation of the apolipoprotein, and the subsequent dissociation of the lipid-apolipoprotein particle from the cell (Oram, supra (1996); Mendez. A.J., J. LipidRes., 38, 1807-1821 (1997); Remaley, supra (1997); Mendez, A.J., J. Lipid Res., 37, 2510-2524 (1996)). Once removed from the cell, the cholesterol-rich HDL particles are transported to the liver and removed from the body as described. Abnormal lipoprotein function and/or metabolism resulting from genetic defect or as a secondary effect of another disorder can have serious biological consequences. In addition to dietary influences, disorders such as diabetes, hypothyroidism, and liver disease can result in elevated plasma levels of LDL-cholesterol and triglycerides. Elevated levels of LDL-cholesterol and triglycerides have been identified as major risk factors associated with the incidence of coronary heart disease, which is the primary cause of death in the United States and other industrialized nations (Hokanson et al., J. Cardiovasc. Risk., 3:213-219 (1996); The Expert Panel, JAMA, 269:3015-3023 (1993)). The accumulation of excess LDL-cholesterol on arterial walls can lead to the formation of atherosclerotic plaques, which play a major role in the development of heart disease. A plaque is believed to form when free radicals released from arterial walls oxidize LDL. According to theory, the oxidized form of LDL triggers an inflammatory response, attracting circulating cells to the site which contribute to the formation of a lipid plaque. Among these are macrophages and other cells that contain scavenger receptors that accumulate cholesterol in an unregulated manner (Brown et al., Ann. Rev. Biochem., 52:223- 261 (1986)). Vast stores of internal cholesterol result in conversion to a foam cell phenotype, which is believed to be a major contributor to the development of vascular lesions. As the plaque builds up, the arterial walls constrict, reducing blood flow to the heart. Interestingly, however, an estimated 60% of heart attacks occur in persons who do not have elevated blood levels of LDL-cholesterol. Of these, an estimated 45% are associated with below average blood levels of HDL-cholesterol, indicating that low HDL-cholesterol level is a significant risk factor for coronary heart disease. In fact, recent studies have indicated that a decreased HDL-cholesterol level is the most common lipoprotein abnormality seen in patients with premature coronary artery disease (Genest J., Circulation, 85:2025-2033 (1992); Genest et al, Arterioscler. Thromb., 13: 1728- 1737 ( 1993)). Although the basis for the inverse association between HDL-cholesterol and coronary heart disease is not well understood, it has been suggested that the cardioprotective role of HDL may stem from its activity relating to the promotion of cholesterol efflux from macrophage foam cells in atherosclerotic lesions. One example of cardiovascular disease associated with low HDL is Tangier disease (TD), a rare genetic disorder characterized by a near or complete absence of circulating HDL. In addition to near zero plasma levels of HDL, patients with TD have a massive deposition and accumulation of cholesteryl esters in several tissues, including tonsils, lymph nodes, liver, spleen, thymus. intestine, and Schwann cells (Fredrickson, D.S., J. Clin. Invest.. 43, 228-236 (1964); Assmann et al., The Metabolic Basis of Inherited Disease, (McGraw-Hill, New York, 1995)). Although the cellular mechanisms have not been previously identified, recent studies have shown that cells from subjects with TD are defective in the process of apolipoprotein- mediated removal of cholesterol and phospholipids (Remaley et al., Arterioscler. Thromb. Vase. Biol., 17, 1813-1821 (1997); Francis et al, J. Clin. Invest., 96, 78-87 (1995); Rogler et al., Arterioscler. Thromb. Vase. Biol., 15, 683-690 (1995)). These results have led to the proposal that the severe HDL deficiency in TD patients stems from the inability of nascent apo A-I to acquire lipids. Because they do not mature into lipid-rich particles, the nascent HDL in TD patients is rapidly catabolized and removed from the plasma, resulting in the near zero levels of circulating HDL (Remaley, supra (1997); Francis, supra (1995); Horowitz et al., J. Clin. Invest., 91, 1743-1752 (1993); Schaefer et al., J. Lip. Res., 22:217-228 (1981)). Other disorders associated with severe premature atherosclerosis and high risk for coronary heart disease resulting from diminished HDL-cholesterol levels are hypoalphalipoproteinemia and familial HDL deficiency syndrome (FHA). Persons with these disorders often have normal LDL-cholesterol and triglyceride levels. In addition, disorders such as diabetes, alcoholism, hypothyroidism, liver disease, and elevated blood pressure can result in diminished plasma levels of HDL-cholesterol, although many of these disorders are also accompanied by elevated LDL-cholesterol and triglceride levels. Current treatments for coronary heart disease have focused primarily on diet manipulations and/or drug therapies aimed at lowering the plasma level of LDL-cholesterol by inhibiting LDL secretion or promoting LDL turnover. Derivatives of fibric acid, such as clofibrate, gemfibrozil, and fenofibrate, promote rapid VLDL turnover by activating lipoprotein lipase. Nicotinic acid reduces plasma levels of VLDL and LDL by inhibiting hepatic VLDL secretion. In addition, HMG-CoaA reductase inhibitors, such as mevinolin, mevastatin, pravastatin, simvastatin, fluvastatin, and lovastatin reduce plasma LDL levels by inhibiting the intracellular synthesis of cholesterol, which causes an increase in the cellular uptake of LDL. In addition, bile acid-binding resins, such as cholestyrine, colestipol and probucol decrease the level of LDL-cholesterol by increasing the catabolism of LDL-cholesterol in the liver. However, many of these therapies are associated with low efficacy and/or side effects that may prevent long-term use. For example, use of HMG-CoaA reductase inhibitors carry a significant risk of toxicity because they inhibit the synthesis of mevalonate, which is required for the synthesis of other important isoprenoid compounds in addition to cholesterol. Also, gemfibrozil and nicotinic acid are associated with serious adverse effects, including renal injury, myopathy, myoglobinuria and intolerable skin flushing and itching. In addition, the role of probucol in treating patients with coronary heart disease is uncertain because its administration results in lower HDL-cholesterol levels as a side effect of reducing LDL-cholesterol. Furthermore, treating patients who have isolated low HDL-cholesterol levels provides a particularly difficult therapeutic challenge. For instance, patients with Tangier disease exhibit a 4- to 6-fold increase in cardiovascular disease even though their LDL levels are already reduced by about 50%. While there is some evidence that gemfibrozil and nicotinic acid may simultaneously elevate HDL levels, in general, therapies aimed at lowering plasma LDL- cholesterol levels are not effective for Tangier patients who suffer from coronary heart disease as a result of diminished HDL levels. Likewise, patients with hypoalphalipoproteinemia, familial HDL deficiency syndrome, or other cardiovascular disease resulting from low levels of HDL will not benefit from therapies aimed at lowering the level of plasma LDL. The problems associated with current therapies for cardiovascular disease stem partially from the fact that the biology involved in the movement of cholesterol in and out of cells is not fully understood. Furthermore, the proteins that play a role in cholesterol movement are not fully known. Therefore, there remains a need for a better understanding of cholesterol cell biology, as well as new methods for treating humans suffering from cardiovascular disease and other disorders associated with hypercholesterolemia. Additionally, there remains a need for new methods of diagnosing cardiovascular disease and new methods of screening patients to identify those at high risk for developing cardiovascular disease. The identification of genes and proteins involved in cholesterol transport would be useful in the development of pharmaceutical agents for the treatment of heart disease and other disorders associated with hypercholesterolemia and atherosclerosis. In addition, the identification of such genes would be useful in the development of screening assays to screen for compounds that regulate the expression of genes associated with cholesterol transport. The identification of such regulatory compounds would be useful in the development of further therapeutic agents. Furthermore, the identification of genes and proteins involved in cholesterol transport would be useful as diagnostic indicators of cardiovascular disease and other disorders associated with hypercholesterolemia. SUMMARY OF THE INVENTION The present invention provides novel polypeptides and polynucleotides involved in cholesterol efflux. Specifically, the present invention provides novel ATP-Binding Cassette (ABCl) polypeptides and novel polynucleotides that encode ABCl polypeptides. The terms "ABCl " and "ABCA1 " are alternative names for the same ATP-Binding Cassette protein and gene. The invention provides ABCl polypeptides, polypeptide fragments, and polypeptide variants. In one preferred embodiment, the present invention provides an isolated polypeptide comprising SEQ ID NO: 2. In another preferred embodiment, the present invention provides an isolated polypeptide comprising an amino acid sequence that has at least 98% identity to SEQ ID NO: 2. The present invention also provides ABCl polypeptides from Tangier disease patients. In one preferred embodiment, the present invention provides an isolated polypeptide comprising SEQ ID NO: 8. In another preferred embodiment, the present invention provides an isolated polypeptide comprising SEQ ID NO: 10. In addition, the present invention provides ABCl polynucleotides, polynucleotide fragments, and polynucleotide variants. In one preferred embodiment, the present invention provides an isolated polynucleotide that encodes the polypeptide comprising SEQ ID NO: 2. In another preferred embodiment, the invention provides an isolated polynucleotide that encodes a polypeptide comprising an amino acid sequence that has at least 98% identity to SEQ ID NO: 2. Also, in other preferred embodiments, the invention provides an isolated polynucleotide comprising a nucleotide sequence that is complementary to a polynucleotide encoding the polypeptide comprising SEQ ID NO: 2 or an isolated polynucleotide comprising a nucleotide sequence that is complementary to a polynucleotide encoding the polypeptide comprising an amino acid sequence that has at least 98% identity to SEQ ID NO: 2. In another preferred embodiment, the present invention provides an isolated ABC 1 polynucleotide comprising SEQ ID NO: 1. In a further preferred embodiment, the present invention provides an isolated polynucleotide comprising nucleotides 291-7074 of SEQ ID NO: 1. In yet another preferred embodiment, the invention provides a polynucleotide comprising a nucleotide sequence that has at least 90% identity with SEQ ID NO: 1. More preferably, the polynucleotide comprises a nucleotide sequence that has at least 95% identity with SEQ ID NO: 1. In other more preferred embodiments, the polynucleotide comprises a nucleotide sequence that has at least 96%, 97%, 98%, or 99% identity to SEQ ID NO: 1. Also, in other preferred embodiments, the present invention provides an isolated polynucleotide comprising a nucleotide sequence that is complementary to the polynucleotide comprising SEQ ID NO: 1 , an isolated polynucleotide comprising a nucleotide sequence that is complementary to a polynucleotide comprising nucleotides 291 -7074 of SEQ ID NO: 1, and an isolated polynucleotide that is complementary to a polynucleotide comprising a nucleotide sequence that has at least 90% identity with SEQ ID NO: 1. The present invention also provides ABCl polynucleotides corresponding to the 5' flanking region of the ABCl gene. In one preferred embodiment, the invention provides an isolated polynucleotide comprising SEQ ID NO: 3. In other preferred embodiments, the invention provides an isolated polynucleotide comprising nucleotides 1-1532, 1080-1643, 1 181- 1643, 1292-1643, or 1394-1532 of SEQ ID NO: 3. Preferably, the isolated polynucleotide comprises nucleotides 1394-1532 of SEQ ID NO: 3. In another preferred embodiment, the invention provides an isolated polynucleotide that hybridizes under stringent conditions to a polynucleotide comprising SEQ ID NO: 3. Also, in other preferred embodiments, the present invention provides an isolated polynucleotide that hybridizes under stringent conditions to a polynucleotide comprising nucleotides 1-1532, 1080-1643, 1 181-1643, 1292-1643, or 1394-1532 of SEQ ID NO: 3. In yet another preferred embodiment of the present invention, an isolated polynucleotide that has at least 80% identity to a polynucleotide comprising SEQ ID NO: 3 is provided. More preferably, the polynucleotide has at least 90% identity to a polynucleotide comprising SEQ ID NO: 3. Even more preferably, the polynucleotide has at least 95% identity to a polynucleotide comprising SEQ ID NO: 3. Also provided in preferred embodiments is an isolated polynucleotide that has at least 80% identity to a polynucleotide comprising nucleotides 1-1532, 1080-1643, 1 181-1643, 1292-1643, or 1394-1532 of SEQ ID NO: 3. More preferably, the polynucleotide has at least 90% identity, and even more preferably at least 95% identity, to a polynucleotide comprising nucleotides 1-1532, 1080-1643, 1 181-1643, 1292-1643, or 1394-1532 of SEQ ID NO: 3. In addition, the present invention provides an isolated polynucleotide comprising a nucleotide sequence that is complementary to the above described 5' flanking ABCl polynucleotides. In one preferred embodiment, the invention provides an isolated polynucleotide comprising a nucleotide sequence that is complementary to a polynucleotide comprising SEQ ID NO: 3. In another preferred embodiment, the present invention provides an isolated polynucleotide comprising a nucleotide sequence that is complementary to a polynucleotide comprising nucleotides 1-1532, 1080-1643, 1 181-1643, 1292-1643, or 1394-1532 of SEQ ID NO: 3. The present invention also provides ABCl polynucleotides corresponding to the 3' flanking region of the ABCl gene. In preferred embodiments, the invention provides an isolated polynucleotide comprising SEQ ID NO: 4, SEQ ID NO: 5. or SEQ ID NO: 6, and the complementary sequences thereof. In other preferred embodiments, the invention provides an isolated polynucleotide that hybridizes under stringent conditions to a polynucleotide comprising SEQ ID NO: 4, SEQ ID NO: 5, or SEQ ID NO: 6, and the complementary sequences thereof. In still other preferred embodiments, the invention provides an isolated polynucleotide that has at least 80% identity to a polynucleotide comprising SEQ ID NO: 4, SEQ ID NO: 5, or SEQ ID NO: 6, and the complementary sequence thereof. More preferably, the polynucleotide has at least 90% identity to a polynucleotide comprising SEQ ID NO: 4, SEQ ID NO: 5, or SEQ ID NO: 6. Even more preferably, the polynucleotide has at least 95% identity to a polynucleotide comprising SEQ ID NO: 4, SEQ ID NO: 5, or SEQ ID NO: 6. In addition, the present invention also provides ABCl polynucleotides from Tangier disease patients. In one preferred embodiment, the present invention provides an isolated polynucleotide encoding the polypeptide comprising SEQ ID NO: 8. In another preferred embodiment, the present invention provides an isolated polynucleotide comprising SEQ ID NO: 7. In yet another embodiment, the present invention provides an isolated polynucleotide encoding the polypeptide comprising SEQ ID NO: 10. In still another preferred embodiment, the present invention provides an isolated polynucleotide comprising SEQ ID NO: 9. The present invention further provides an isolated polynucleotide comprising a nucleotide sequence that is complementary to the described polynucleotides. In another aspect, the present invention provides a composition comprising any of the above described polynucleotides and a suitable carrier. In one preferred embodiment, the present invention provides a composition comprising an isolated polynucleotide encoding a polypeptide comprising SEQ ID NO: 2, a polynucleotide comprising SEQ ID NO: 1, a polynucleotide comprising nucleotides 291-7074 of SEQ ID NO: 1 , or a polynucleotide encoding a polypeptide comprising an amino acid sequence that has at least 98% identity to SEQ ID NO: 2, and a suitable carrier. In another preferred embodiment, the composition comprises an isolated polynucleotide comprising a nucleotide sequence that has at least 90% identity with a polynucleotide comprising SEQ ID NO: 1 and a suitable carrier. In other preferred embodiments, the composition comprises an isolated polynucleotide comprising SEQ ID NO: 3 or an isolated polynucleotide comprising nucleotides 1-1532, 1080-1643, 1 181-1643, 1292-1643, or 1394-1532 of SEQ ID NO: 3 and a suitable carrier. In still other preferred embodiments, the invention provides a composition comprising a polynucleotide that hybridizes under stringent conditions to a polynucleotide comprising SEQ ID NO: 3, or a polynucleotide comprising nucleotides 1-1532, 1080-1643, 1181-1643, 1292-1643, or 1394-1532 of SEQ ID NO: 3, as well as a composition comprising a polynucleotide that has at least 80% identity to a polynucleotide comprising SEQ ID NO: 3, or a polynucleotide comprising nucleotides 1-1532, 1080-1643, 1 181-1643, 1292-1643, or 1394-1532 of SEQ ID NO: 3 and a suitable carrier. Also provided by the present invention is a composition comprising an isolated polynucleotide comprising a nucleotide sequence that is complementary to any of the described polynucleotides and a suitable carrier. In addition, the present invention provides recombinant vectors and host cells comprising any of the described ABCl polynucleotide sequences. In one preferred embodiment, the present invention provides a recombinant vector comprising an isolated polynucleotide encoding a polypeptide comprising SEQ ID NO: 2, an isolated polynucleotide comprising SEQ ID NO: 1, an isolated polynucleotide comprising nucleotides 291-7074 of SEQ ID NO: 1, or an isolated polynucleotide encoding the polypeptide comprising an amino acid sequence that has at least 98% identity to SEQ ID NO: 2. In another preferred embodiment, the recombinant vector comprises an isolated polynucleotide comprising a nucleotide sequence that has at least 90% identity, and more preferably at least 95% identity, with a polynucleotide comprising SEQ ID NO: 1. In still another preferred embodiment, the recombinant vector comprises an isolated polynucleotide comprising SEQ ID NO: 7 or SEQ ID NO: 9. The present invention further provides a recombinant vector comprising an isolated polynucleotide comprising a nucleotide sequence that is complementary to any of the described polynucleotides. In yet another preferred embodiment, the recombinant vector comprises any of the described polynucleotides and further comprises a heterologous promoter polynucleotide. One suitable heterologous promoter is a cytomegalovirus promoter. In a particularly preferred embodiment, the recombinant vector is pCEPhABCl . The present invention also provides a recombinant vector comprising an isolated polynucleotide comprising an ABCl 5' flanking sequence. In one preferred embodiment, the invention provides a recombinant vector comprising an isolated polynucleotide comprising SEQ ID NO: 3 or an isolated polynucleotide comprising nucleotides 1-1532, 1080-1643, 1 181-1643, 1292-1643, or 1394-1532 of SEQ ID NO: 3. In still other preferred embodiments, the invention provides a recombinant vector comprising a polynucleotide that hybridizes under stringent conditions to the polynucleotide of SEQ ID NO: 3, or a polynucleotide comprising nucleotides 1- 1532, 1080-1643, 1 181-1643, 1292-1643, or 1394-1532 of SEQ ID NO: 3, as well as a recombinant vector comprising a polynucleotide that has at least 80% identity to these polynucleotides. The present invention further provides a recombinant vector comprising an isolated polynucleotide comprising a nucleotide sequence that is complementary to any of the described polynucleotides. In yet another preferred embodiment, the recombinant vector comprises any of the described polynucleotides and further comprises at least one polynucleotide encoding a heterologous polypeptide. Suitable heterologous polypeptides include luciferase, β- galactosidase, chloramphenicol acetyl transferase transferase, and green fluorescent proteins. Preferably, the heterologous polypeptide is a luciferase protein. In a particularly preferred embodiment, the recombinant vector is pAPRl . In addition, the present invention provides host cells comprising any of the described recombinant vectors. The present invention further provides compositions comprising any of the described recombinant vectors and a suitable carrier. The present invention also provides methods for producing the ABCl protein in a mammalian host cell as well as methods for expressing the ABC 1 protein in a mammalian subject. The method for producing an ABCl protein in a mammalian host cell comprises the steps of: (a) transfecting the mammalian host cell with a recombinant expression vector comprising a polynucleotide encoding ABCl in an amount sufficient to produce a detectable level of ABCl protein, and (b) purifying the produced ABCl protein. The method for expressing ABCl protein in a mammalian subject comprises the step of administering to a mammalian subject a recombinant expression vector comprising a polynucleotide encoding ABCl in an amount sufficient to express ABCl protein in the mammalian subject. In addition, the present invention provides compositions and methods suitable for increasing cholesterol efflux from cells of a mammalian subject. In one preferred embodiment, the method comprises administering to the mammalian subject a recombinant expression vector comprising a polynucleotide encoding ABC 1 in an amount sufficient to increase cholesterol efflux from the cells. Suitable recombinant expression vectors include vectors comprising an isolated polynucleotide encoding a polypeptide comprising SEQ ID NO: 2, an isolated polynucleotide comprising SEQ ID NO: 1 , an isolated polynucleotide comprising nucleotides 291-7074 of SEQ ID NO: 1 , and an isolated polynucleotide encoding the polypeptide comprising an amino acid sequence that has at least 98% identity to SEQ ID NO: 2. Preferred expression vectors include viral vectors, especially adenoviral vectors and lentiviral vectors. In other embodiments, the invention provides non-viral delivery systems, including DNA-ligand complexes, adenovirus-ligand-DNA complexes, direct injection of DNA, CaPO 4 precipitation, gene gun techniques, electroporation, liposomes and lipofection. In another preferred embodiment, the method for increasing cholesterol efflux from cells of a mammalian subject comprises administering to the mammalian subject a therapeutic amount of a compound that increases the expression of ABCl in the cells. One suitable method comprises administering to the mammalian subject a cAMP analogue. Suitable cAMP analogues include 8-bromo cAMP, N6-benzoyl cAMP, and 8-thiomethyl cAMP. Another suitable method comprises administering to the mammalian subject a compound that increases the synthesis of cAMP, e.g. forskolin. Yet another suitable method comprises administering to the mammalian subject a compound that inhibits the degradation of cAMP, such as a phosphodiesterase inhibitor. Suitable phosphodiesterase inhibitors include rolipram, theophylline, 3-isobutyl-l- methylxanthine, R020-1724, vinpocetine, zaprinast, dipyridamole, milrinone, amrinone, pimobendan, cilostamide, enoximone, peroximone, and vesnarinone. In addition, another suitable method for increasing cholesterol efflux from cells of a mammalian subject comprises administering to the mammalian subject a least one ligand for a nuclear receptor in an amount sufficient to increase cholesterol efflux. Suitable ligands include LXR, RXR, FXR, SXR and PPAR ligands. In one preferred embodiment, the method comprises administering to a mammalian subject a ligand for an LXR nuclear receptor. Suitable LXR ligands include 20(S) hydroxycholesterol, 22(R) hydroxycholesterol, 24(S) hydroxycholesterol, 25-hydroxycholesterol, and 24(S), 25 epoxycholesterol. Preferably, the LXR ligand is 20(S) hydroxycholesterol. In another preferred embodiment, the method comprises administering to a mammalian subject a ligand for an RXR nuclear receptor. Suitable RXR ligands include 9-cis retinoic acid, retinol, retinal, all-trans retinoic acid, 13-cis retinoic acid, acitretin, fenretinide, etretinate, CD 495, CD564, TTNN, TTNNPB, TTAB, and LGD 1069. Preferably, the RXR ligand is 9-cis retinoic acid. In another preferred embodiment, the method comprises administering to a mammalian subject a ligand for a PPAR nuclear receptor. One suitable ligand is a ligand selected from the class of thiazolidinediones. In yet another preferred embodiment, the method comprises administering at least two ligands for a nuclear receptor. In a particularly preferred embodiment, the ligands are 20(S) hydroxycholesterol and 9-cis retinoic acid. In addition, another suitable method for increasing cholesterol efflux from cells of a mammalian subject comprises administering to the mammalian subject an eicosanoid in an amount sufficient to increase cholesterol efflux. Suitable eicosanoids include prostaglandin E2, prostaglandin J2, and prostacyclin (prostaglandin 12). In another embodiment, the present invention provides a method for increasing cholesterol efflux from cells of a mammalian subject comprising administering to the mammalian subject a compound that increases ABCl activity in an amount sufficient to increase cholesterol efflux from the cells. The present invention also provides methods suitable for increasing the gene expression of ABCl in a mammalian subject. In one preferred embodiment, the method comprises administering to the mammalian subject at least one ligand for a nuclear receptor in anamount sufficient to increase the gene expression of ABCl. Suitable ligands include ligands for LXR, RXR, FXR, SXR, and PPAR nuclear receptors. In another preferred embodiment, the method comprises administering to the mammalian subject a cAMP analogue in an amount sufficient to increase the gene expression of ABC 1. In yet another preferred embodiment, the method comprises administering to the mammalian subject a compound that increases the synthesis of cAMP in an amount sufficient to increase the gene expression of ABCl. In addition, the present invention provides a method for screening a test compound for ABCl expression modulating activity comprising the steps of: (a) operatively linking a reporter cDNA with an expression modulating portion of the mammalian ABCl gene to produce a recombinant reporter construct; (b) transfecting the recombinant reporter construct into a population of host cells; (c) assaying the level of reporter gene expression in a sample of the host cells; (d) contacting the host cells with the test compound being screened; (e) assaying the level of reporter gene expression in a sample of the host cells after contact with the test compound; and (f) comparing the relative change in the level of reporter gene expression caused by exposure to the test compound, thereby determining the ABCl expression modulating activity. The recombinant reporter construct comprises a reporter gene operatively linked to an expression modulating portion of the mammalian ABCl gene, such as any of the ABCl 5' flanking region sequences provided by the present invention. In one preferred embodiment, the expression modulating portion of the ABCl gene comprises SEQ ID NO: 3. In another preferred embodiment, the expression modulating portion of the ABCl gene comprises nucleotides 1- 1532, 1080-1643, 1 181-1643, 1292-1643. 1394-1643, or 1394-1532 of SEQ ID NO: 3. Suitable reporter cDNAs include luciferase, β-galactosidase, chloramphenicol acetyl transferase, and green fluorescent protein cDNA. Preferably, the host cell is a mammalian cell. In a particularly preferred embodiment of the method, the recombinant reporter construct is pAPRl . Also provided by the present invention is a method for screening a test compound to determine whether the test compound promotes ABCl -mediated cholesterol efflux from cells in culture comprising the steps of: (a) assaying the level of cholesterol efflux in a sample of mammalian cells maintained in culture to determine a control level of cholesterol efflux; (b) contacting the cells with the test compound being screened; (c) assaying the level of cholesterol efflux in a sample of cells after contact with the test compound; (d) assaying the level of ABCl - mediated cholesterol efflux in a sample of cells after contact with the test compound, thereby determining whether the test compound promotes ABCl -mediated cholesterol efflux from cells in culture. The cells can be derived from primary cultures or a cell line. Suitable cells for screening the test compound include fibroblast, macrophage, hepatic, and intestinal cell lines. Preferably, the cell line is RAW 264.7. In one preferred embodiment, the ABCl -mediated cholesterol efflux is measured using an anti-ABC 1 antibody that inhibits the activity of ABCl upon binding. In another preferred embodiment, the ABC 1 -mediated cholesterol efflux is measured using an antisense ABCl polynucleotide. In a particularly preferred embodiment, the antisense polynucleotide comprises SEQ ID NO: 57. In addition, the present invention provides methods for detecting the comparative level of ABCl expression in cells of a mammalian subject. Such methods can be used to determine the susceptibility of a subject to coronary heart disease. A method for detecting the comparative level of ABCl expression in cells of a mammalian subject is provided which comprises (a) obtaining a cell sample from the mammalian subject, (b) assaying the level of ABCl mRNA expression in the cell sample; and (c) comparing the level of ABCl mRNA expression in the cell sample with a pre-determined standard level of ABCl mRNA expression, thereby detecting the comparative level of ABCl gene expression in the cells of a mammalian subject. Suitable methods for measuring the level of ABC 1 mRNA expression include, for example, RT-PCR, northern blot, and RNAse protection assay. The present invention also provides methods for detecting the comparative level of ABCl protein in cells of a mammalian subject. Such methods can be used to determine the susceptibility of a subject to coronary heart disease. A method for detecting the comparative amount of ABCl protein in the cells of a mammalian subject is provided which comprises (a) obtaining a cell sample from the mammalian subject, (b) assaying the amount of ABCl protein in the cell sample, and (c) comparing the amount of ABCl protein in the cell sample with a predetermined standard amount of ABCl protein, thereby detecting the comparative level of ABCl protein in the cells of the mammalian subject. The amount of ABCl protein can be determined using various immunoassays available in the art. For example, the amount of ABCl protein can be determined by (a) contacting the cell sample with a population of anti-ABC 1 antibodies and (b) detecting the specific-binding ABCl antibodies associated with the sample. Suitable methods for detecting ABCl antibodies include western blotting, immunoprecipitation, and FACS. In another aspect, the present invention provides antibodies that bind specifically to the described ABCl polypeptides. In one preferred embodiment, the present invention provides an isolated antibody that binds specifically to an isolated polypeptide comprising SEQ ID NO: 2. In another preferred embodiment, the invention provides an isolated antibody that bind specifically to an isolated polypeptide comprising an amino acid sequence that has at least 98% identity with SEQ ID NO: 2. The antibody can be a monoclonal antibody or the antibody can be a polyclonal antibody. In yet another embodiment, the antibody, upon binding to an ABCl polypeptide, inhibits the cholesterol transport activity of the ABC 1 polypeptide. In addition, the present invention provides kits suitable for screening a compound to determine the ABCl expression modulating activity of the compound comprising a reporter cDNA operatively linked to an expression modulating portion of the mammalian ABC 1 gene in an amount sufficient for at least one assay and instructions for use. In one preferred embodiment, the kit further comprises means for detecting the reporter gene. In another preferred embodiment, the expression modulating portion of the mammalian ABCl gene comprises SEQ ID NO: 3. In yet another preferred embodiment, the expression modulating portion of the mammalian ABC 1 gene comprises nucleotides 1-1532, 1080-1643, 1 181-1643, 1292-1643, 1394-1643, or 1394-1532 of SEQ ID NO: 3. Suitable reporter cDNAs include luciferase, β-galactosidase, chloramphenicol acetyl transferase, and green fluorescent protein cDNA. Preferably, the reporter cDNA is luciferase. In a particularly preferred embodiment of the method, the recombinant reporter construct is pAPRl . The present invention also provides kits suitable for screening a compound to determine whether the compound modulates ABCl -dependent cholesterol efflux. In one preferred embodiment, the kit comprises an inactivating anti-ABC 1 antibody in an amount sufficient for at least one assay and instructions for use. In another preferred embodiment, the kit comprises an antisense ABCl oligonucleotide in an amount sufficient for at least one assay and instructions for use. In a particularly preferred embodiment, the antisense ABCl oligonucleotide comprises SEQ ID NO: 53. BRIEF DESCRIPTION OF THE FIGURES Figure 1A-D is a graphical representation showing the results of control cholesterol efflux and cholesterol efflux in the presence of HDL and apo A-I from normal fibroblast cells (1A, C) and fibroblast cells from Tangier disease patients (IB, D). The open circles represent the cholesterol efflux from cells that were not exposed to HDL or apo A-I, the closed circles represent the cholesterol efflux from cells exposed to HDL, and the closed diamonds represent the cholesterol efflux from cells exposed to apo A-I; Figure 2 is a graphical representation of a gene expression microarray analysis showing a comparison of the gene expression found in cells from a Tangier patient (TD1 ) and that found in normal cells, whereby a total of 58,800 human cDNAs were hybridized with cDNA prepared from mRNA of cAMP-treated TD1 cells cDNA (labeled with Cy3 dye) and with cDNA prepared from mRNA of cAMP -treated normal cells (labeled with Cy5 dye); Figure 3 is a schematic diagram showing a restriction map of the recombinant expression vector pCEPhABCl, which contains the open reading frame of the human ABCl gene; Figure 4 is a schematic diagram of the gene structure of human ABCl, showing a comparison between the published human ABCl amino acid sequence (GenBank, Accession # AJ012376) and the presently disclosed and claimed human ABCl amino acid sequence ("CVT") which has sixty additional amino acids at the N-terminal end; Figure 5 is a graphical representation showing the inhibitory effect that ABC 1 transport inhibitors 4,4-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) and sulphobromophtaleine (BSP) have on apo A-I-mediated cholesterol efflux, wherein the open circles indicate the apo A- I-mediated cholesterol in the presence of BSP and the closed circles indicate the apo A-I- mediated cholesterol in the presence of DIDS; Figure 6 is a graphical representation showing the inhibitory effect of an antisense ABC 1 oligonucleotide on apo A-I-mediated cholesterol efflux, showing the apo A-I-mediated cholesterol efflux in cells incubated without antisense oligonucleotide, the apo A-I-mediated cholesterol efflux in cells exposed to 30 μM β-globin antisense oligonucleotide, and the apo A-I- mediated cholesterol efflux in cells exposed to 30 μM ABCl antisense oligonucleotide; Figure 7 is a graphical representation demonstrating the stimulation of apo A-I-mediated cholesterol efflux caused by overexpression of the ABC l gene using RAW 264.7 mouse macrophage cells stably transfected with an expression plasmid for human ABCl (pCEPhABCl), showing the apo A-I-mediated cholesterol efflux in control parental cells (no pCEPhABCl) and the apo A-I-mediated cholesterol efflux in clonal cells transfected with pCEPhABCl (L3, L5, L6); Figure 8 is a graphical representation of reverse transcription polymerase chain reaction (RT-PCR) analyses showing the level of ABCl gene expression in normal cells and cells from Tangier's disease patients (TD 1 and TD2) that have been either exposed to albumin (closed bars), exposed to 8-Br-cAMP (open bars), cholesterol-loaded (shaded bars), or cholesterol-loaded and subsequently exposed to apo A-I (hatched bars); Figure 9 is a graphical representation of the results of RT-PCR analyses showing the level of ABCl gene expression in RAW 264.7 cells exposed to either ethanol (0.1% v/v), 9-cis retinoic acid (9-cis RA; lOμM), 20(S) hydroxycholesterol (20(S)-OH; l OμM), or 9-cis RA and 20(S)-OH (1 OμM each); Figure 10 shows the results of immunoprecipitation analyses indicating the level of cell- surface ABCl protein found in normal fibroblasts (NL1, 10A) and fibroblasts from a Tangier's disease patient (TD1, 10B) in the presence of either no additives (control), 8-Br-cAMP (ImM), cholesterol (30 μg/ml), or cholesterol and 8-Br-cAMP (30μg/ml and ImM, respectively); Figure 1 1 is a schematic diagram showing a restriction map of the recombinant expression vector pAPRl, which contains the 5' flanking region of the ABCl gene positioned upstream of the open reading frame of the luciferase reporter gene; Figure 12 is a graphical representation showing the level of luciferase reporter gene expression induced in RAW 264.7 cells transfected with pAPRl in the presence of either EtOH (control), 20(S)-OH ( 1 OμM), 9-cis RA ( 1 OμM), or both 20(S)-OH and 9-cis RA ( 1 OμM each); Figure 13 is a schematic diagram of the 5' flanking region of the ABCl gene. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with the present invention, novel polypeptides that increase the cholesterol efflux from cells are provided. In particular, the present invention provides novel ATP-Binding Cassette 1 (ABCl) polypeptides that have been shown to increase cholesterol efflux. ABCl is a member of the family of ATP -binding cassette proteins that reside in cell membranes and utilize ATP hydrolysis to transport a wide variety of substrates across the plasma membrane. It should be noted that the terms "ABCl " and "ABCA1 " both refer to the same ATP- binding cassette protein. The term "ABCA1 " was introduced in 1999 by a nomenclature committee and has received limited acceptance in the field. To date, more than 30 members of this family have been identified in the human genome. These homologous proteins contain channel-like structures through which molecules are transported through the cell membrane and one or two domains which bind to ATP to couple energy generating ATP-hydrolysis to transport. Family members include the multidrug resistance factors (MDR/P-glycoproteins; Chen et al., Cell, 47: 381-389 (1986); Stride et al., Mol. Pharmacol, 49:962-971 (1996)), transporters associated with antigen presentation (Neefjes et al., Science, 261 :769-771 (1993); Shepherd et al., Cell, 74:577-584 (1993)), and the cystic fibrosis transmembrane conductance regulator (Chang et al, J. Biol. Chem., 269: 18572-18575 (1994); Rommens et al., Science, 245: 1059- 1065 (1989)). Members of the ABC transporter family are generally composed of 4 domains found within two symmetric halves that are linked by a long charged region and a highly hydrophobic segment. Each half contains a hydrophobic domain, containing 6 transmembrane segments and a hydrophilic nucleotide binding domain containing highly conserved Walker A and B sequence motifs typical of many ATPases (Hyde et al., Nature, 346:362-365 (1990); Luciani et al., Genomics, 21 : 150-159 (1994)). The transporter activity is dependent on the interaction with ATP at the nucleotide binding domains and by regulation via phosphorylation of residues in the region linking the two symmetric halves (Becq et al, J. Biol. Chem., 272: 2695- 2699 (1997)). Several lines of evidence described herein identified ABCl as a pivotal protein in the apolipoprotein-mediated mobilization of intracellular cholesterol stores. First, the studies presented herein showed that ABCl is defective in Tangier disease, a genetic disorder characterized by abnormal HDL-cholesterol metabolism. As shown previously, and herein at Example 1, the genetic defect in Tangier disease causes a defect in the pathway of apolipoprotein mediated efflux of cholesterol from within cells, resulting in significantly decreased cholesterol efflux activity and low HDL-cholesterol levels (Oram et al., J. LipidRes., 37:2743-2491 (1996); Francis et al., J. Clin. Invest., 96: 78-87 (1995)). Genetic linkage analysis of families with Tangier disease assigned the defective gene to an interval on chromosome 9q31 (Rust et al., Nature Genetics, 20: 96-98 (1998)). A search of public databases revealed that the ABCl gene was localized to chromosome 9q22-9q31 , which is broader than, but includes the interval revealed in Rust et al. (Luciani et al., supra (1994)). Based on that data, radiation hybrid mapping of the human ABCl gene was performed, which placed the gene between two markers squarely within the 7-cM region of human chromosome 9q31 reported by Rust et al. In addition, as shown in Example 2. microarray analysis revealed that the ABCl gene is 2.5-fold underexpressed in Tangier patient cells as compared with normal cells. These studies identified the defective gene in Tangier disease as ABC 1. In addition, further studies presented herein linked ABCl activity to cholesterol efflux activity. First, studies showed that inhibitors of ABCl transport activity, such as 4,4-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) and sulphobromophtaleine (BSP), also inhibited apoAI-mediated cholesterol efflux from fibroblast cells (see Example 6). Also, inhibition of ABCl gene expression, using an antisense ABCl oligonucleotide, was shown to inhibit apoAI-mediated cholesterol efflux from fibroblast cells (Example 7). In contrast, transfection studies, in which the ABCl gene was transfected into mouse monocyte cells, showed that overexpression of ABCl results in an increase in apoAI- mediated efflux (Example 8). Finally, RT-PCR performed using wildtype and Tangier patient mRNA revealed that ABC 1 mRNA expression is regulated by cellular conditions related to cholesterol efflux in normal skin fibroblast cells, but not in Tangier patient fibroblasts (Example 9). Based on these findings, it was determined that ABCl plays a major role in cholesterol efflux. It is postulated that ABCl plays a role in the translocation of intracellular cholesterol to the outer leaflet of the plasma membrane. Deficient transport of intracellular cholesterol due to a lack of ABCl or defective ABCl results in a lack of cholesterol in specific membrane domains with which apoAI and other apolipoproteins specifically interact (Stangl et al., J. Biol. Chem., 273: 31002-31008 (1998); Babitt et al, J. Biol. Chem.. 272: 13242-13249 (1997))). The failed delivery of cholesterol to apoAI leads to the formation of cholesterol-deficient HDL particles that are rapidly removed from the plasma. (Bojanovski et al., J. Clin. Invest., 80: 1742-1747 (1987)). Definitions The following definitions are provided to facilitate understanding of certain terms used throughout this specification. In the present invention, "isolated" refers to material removed from its original environment (e.g., the natural environment if it is naturally occurring), and thus is altered "by the hand of man" from its natural state. For example, an isolated polynucleotide could be part of a vector or a composition of matter, or could be contained within a cell, and still be "isolated" because that vector, composition of matter, or particular cell is not the original environment of the polynucleotide. As used herein, the term "polynucleotide(s)" is defined to encompass DNA and RNA of both synthetic and natural origin. The polynucleotide may exist as single- or double-stranded DNA or RNA, or an RNA/DNA heteroduplex. Thus, the polynucleotide of the present invention can be composed of any polyribonucleotide or polydeoxribonucleotide, which may be unmodified RNA or DNA or modified RNA or DNA. For example, polynucleotides can be composed of single- and double-stranded DNA, DNA that is a mixture of single- and double- stranded regions, or single, double-, and triple-stranded regions, single- and double-stranded RNA, and RNA that is mixture of single- and double-stranded regions, hybrid molecules comprising DNA and RNA that may be single-stranded or, more typically, double-stranded or triple-stranded, or a mixture of single- and double-stranded regions. In addition, the polynucleotide can be composed of triple-stranded regions comprising RNA or DNA or both RNA and DNA. A polynucleotide may also contain one or more modified bases or DNA or RNA backbones modified for stability or for other reasons. "Modified" bases include, for example, tritylated bases and unusual bases such as inosine. A variety of modifications can be made to DNA and RNA; thus, "polynucleotide" embraces chemically, enzymatically, or metabolically modified forms of polynucleotides. The term "polynucleotide(s)" also embraces short polynucleotides often referred to as oligonucleotide(s). The term "polypeptide(s)" refers to any peptide or protein comprising two or more amino acids joined to each other by peptide bonds or modified peptide bonds. "Polypeptide" refers to both short amino acid sequences, commonly referred to as peptides, as well as longer amino acid sequences, generally referred to as proteins. The polypeptide may contain amino acids other than the 20 gene encoded amino acids. Moreover, the polypeptide may be modified either by natural processes, such as processing and other post-translational modifications, or by chemical modification techniques, which are well-known in the art. A given polypeptide may contain many types of modifications. Also, the same type of modification may be present in the same or varying degree at one or more sites in the polypeptide. Modifications may occur anywhere in the polypeptide, including the peptide backbone, the amino acid side-chains, and the amino or carboxyl termini. Modifications include, but are not limited to, acetylation, acylation, ADP- ribosylation, amidation, formylation, gamma-carboxylation, glycosylation, hydroxylation, iodination, methylation, myristoylation, oxidation, phosphorylation, prenylation, sulfation, and selenoylation, as well as the covalent attachment of a nucleotide or nucleotide derivative, a lipid or lipid derivative, or a phosphotidylinositol. Other modifications include cross-linking, cyclization, formation of pyroglutamate, GPI anchor formation, proteolytic processing, racemization, and t-RNA-mediated addition of amino acids, such as arginylation and ubiquitination. See, for example, Proteins- Structure and Molecular Properties, 2 nd Ed.. T.E. Creighton, W.H. Freedman and Co., New York (1993); Wold. F., Posttranslational Protein Modification: Perspectives and Prospects, in Posttranslational Covalent Modification of Proteins, B. C. Johnson, Ed., Academic Press, New York (1983); Seifter et al., Meth. Enzymol.. 182: 626- 646 (1990); and Rattan et al., Protein Synthesis: Posttranslational Modifications and Aging, Ann. N.Y. Acad. Sci., 663: 48-62 (1992)). The polypeptides of the invention can be prepared in any suitable manner. Such polypeptides include isolated naturally occurring polypeptides, recombinantly produced polypeptides. synthetically produced polypeptides, or polypeptides produced by a combination of these methods. Means for preparing such polypeptides are well understood in the art. A "polynucleotide" of the present invention also includes those polynucleotides capable of hybridizing, under stringent hybridization conditions, to SEQ ID NO:3 or nucleotides 1-1532, 1080-1643, 1 181-1643, 1292-1643, 1394-1643, or 1394-1532 of SEQ ID NO: 3, or the complements thereof. A polynucleotide of the present invention also includes those polynucleotides capable of hybridizing, under stringent hybridization conditions, to SEQ ID NO:4, SEQ ID NO: 5, or SEQ ID NO: 6 or the complements thereof. "Stringent hybridization conditions" refers to an overnight incubation at 42° C in a solution comprising 50% formamide, 5x SSC (750 mM NaCl, 75 mM sodium citrate), 50 mM sodium phosphate (pH 7.6), 5x Denhardt's solution, 10% dextran sulfate, and 20 μg/ml denatured, sheared salmon sperm DNA, followed by washing the filters in 0. Ix SSC at about 65°C. As used herein, the term "complementary" refers to the hybridization or base pairing between nucleotides, such as, for example, between the two strands of a double-stranded polynucleotide or between an oligonucleotide primer and a primer binding site on a single- stranded polynucleotide to be amplified or sequenced. Two single-stranded nucleotide molecules are said to be complementary when the nucleotides of one strand, optimally aligned with appropriate nucleotide insertions, deletions or substitutions, pair with at least about 80% of the nucleotides of the other strand. "Identity", as known in the art, is a relationship between two or more polypeptide sequences or two or more polynucleotide sequences, as determined by comparing the sequences. "Identity" or "similarity" also has an art-recognized meaning that refers to the degree of sequence relatedness between polypeptide or polynucleotide sequences, as determined by the match between strings of such sequences. "Identity"and "similarity" can be calculated using a number of well known methods, including those published in Computational Molecular Biology, Lesk, A.M.. ed., Oxford University Press, New York, (1988); Biocomputing: Informatics and Genome Projects, Smith, D.W., ed., Academic Press, New York, (1993); Computer Analysis of Sequence Data, Part I, Griffin, A.M., and Griffin, H.G., eds., Humana Press, New Jersey, (1994); Sequence Analysis in Molecular Biology, von Heinje, G., Academic Press, ( 1987); and Sequence Analysis Primer, Gribskov, M. and Devereux, J., eds., M Stockton Press, New York, (1991); and Carillo, H., and Lipton, D., SIAM J Applied Math 48:1073 (1988)). Methods commonly employed to determine identity or similarity between two sequences include, but are not limited to, those disclosed in "Guide to Huge Computers," Martin J. Bishop, ed., Academic Press, San Diego, (1994), and Carillo, H., and Lipton, D., SIAM J Applied Math 48: 1073 (1988). Preferred methods to determine identity are designed to give the largest match between the sequences tested. Methods for aligning polynucleotides or polypeptides are codified in computer programs, including the GCG program package (Devereux, J., et al., Nucleic Acids Research (1984) 12(1):387 (1984)), BLASTP, BLASTN, FASTA (Atschul, S.F. et al, J. Molec. Biol. 215:403 (1990), Bestfit program (Wisconsin Sequence Analysis Package, Version 8 for Unix, Genetics Computer Group, University Research Park, 575 Science Drive, Madison, WI 5371 1 (using the local homology algorithm of Smith and Waterman, Advances in Applied Mathematics 2:482-489 (1981)). When using any of the sequence alignment programs to determine whether a particular sequence is, for instance, 90% identical to a reference sequence, the parameters are set so that the percentage of identity is calculated over the full length of the reference polypeptide or polynucleotide and that gaps in identity of up to 10% of the total number of nucleotides in the reference polynucleotide are allowed. A preferred method for determining the best overall match between a query sequence (a sequence of the present invention) and a subject sequence, also referred to as a global sequence alignment, can be determined using the FASTDB computer program based on the algorithm of Brutlag et al. (Comp. App. Biosci. 6:237-245 (1990)). The term "sequence" includes nucleotide and amino acid sequences. In a sequence alignment the query and subject sequences are either both nucleotide sequences or both amino acid sequences. The result of said global sequence alignment is in percent identity. Preferred parameters used in a FASTDB search of a DNA sequence to calculate percent identity are: Matrix=Unitary, k-tuple=4, Mismatch Penalty=l, Joining Penalty=30, Randomization Group Length=0, and Cutoff Score=l, Gap Penalty=5, Gap Size Penalty 0.05, and Window Size=500 or query sequence length in nucleotide bases, whichever is shorter. Preferred parameters employed to calculate percent identity and similarity of an amino acid alignment are: Matrix=PAM 150, k-tuple=2, Mismatch Penalty= 1, Joining Penalty=20, Randomization Group Length=0, Cutoff Score=l, Gap Penalty=5, Gap Size Penalty=0.05, and Window Size=500 or query sequence length in amino acid residues, whichever is shorter. As an illustration, a polynucleotide having a nucleotide sequence of at least 90% "identity" to a sequence contained in SEQ ID NO: 1 means that the nucleotide sequence of the polynucleotide is identical to a sequence contained in SEQ ID NO: 1 except that the polynucleotide sequence may include up to ten point mutations per each 100 nucleotides of the total length of SEQ ID NO: 1. In other words, to obtain a polynucleotide comprising a nucleotide sequence that has at least 90% identity to SEQ ID NO: 1, up to 10% of the nucleotides in the sequence contained in SEQ ID NO: 1 can be deleted, inserted, or substituted with other nucleotides. These changes may occur anywhere throughout the polynucleotide, and may be interspersed either individually among nucleotides or in one or more contiguous groups within SEQ ID NO: 1. Similarly, a polypeptide having an amino acid sequence of at least 98% "identity" to a sequence contained in SEQ ID NO: 2 means that the amino acid sequence of the polypeptide is identical to a sequence contained in SEQ ID NO: 2 except that the polypeptide sequence may include up to 2 amino acid alterations per each 100 amino acids of the total length of SEQ ID NO: 2. In other words, to obtain a polypeptide having an amino acid sequence at least 98% identical to SEQ ID NO: 2, up to 2% of the amino acid residues in the sequence contained in SEQ ID NO: 2 can be deleted, inserted, or substituted with other amino acid residues. These changes may occur anywhere throughout the polypeptide. and may be interspersed either individually among residues or in one or more contiguous groups within SEQ ID NO: 2. "A polypeptide having biological activity" refers to a polypeptide exhibiting activity similar, but not necessarily identical, to an activity of a polypeptide of the present invention (e.g. cholesterol transport activity), as measured in a particular biological assay, with or without dose dependency. In the case where dose dependency does exist, it need not be identical to that of the polypeptide, but rather substantially similar to the dose-dependence in a given activity as compared to the polypeptide of the present invention (i.e., the candidate polypeptide will exhibit greater activity or not more than about 25-fold less and, preferably, not more than about tenfold less activity, and most preferably, not more than about three-fold less activity relative to the polypeptide of the present invention.). "Polypeptide variant" refers to a polypeptide differing from the ABCl polypeptide of the present invention, but retaining essential properties thereof. Generally, variants are overall closely similar, and. in many regions, identical to the polypeptide comprising SEQ ID NO: 2. Preferably, the polypeptide variant retains its biological activity, i.e., cholesterol transport activity. Variants include, but are not limited to, splice variants and allelic variants, as well as addition, deletion, and substitution variants. Likewise, "polynucleotide variant" refers to a polynucleotide differing from the polynucleotide of the present invention, but retaining essential properties thereof. The variants may contain alterations in the coding regions, non-coding regions, or both. Thus, for example, an ABCl polynucleotide variant has a nucleotide sequence that differs from that of SEQ ID NO: 1, but encodes a polypeptide that has cholesterol transport activity. Also, for example, a polynucleotide variant has a nucleotide sequence that differs from that of SEQ ID NO: 3, but retains promoter activity. Especially preferred are polynucleotide variants containing alterations that produce silent substitutions, additions, or deletions, but do not alter the properties or activities of the encoded polypeptide. Nucleotide variants produced by silent substitutions due to the degeneracy of the genetic code are preferred. Moreover, variants in which 10-20, 5-10, 1-5, or 1-2 amino acids are substituted, deleted, or added in any combination are also preferred. Polynucleotide variants can be produced for a variety of reasons, e.g., to optimize codon expression for a particular host (e.g. changing codons in the human mRNA to those preferred by a bacterial host such as E. coli). "Allelic variants" are naturally-occurring variants that refer to one of several alternate forms of a gene occupying a given locus on a chromosome of an organism. (Genes II, Lewin, B., ed., John Wiley & Sons, New York (1985).) These allelic variants can vary at either the polynucleotide and/or polypeptide level. Alternatively, non-naturally occurring variants may be produced by mutagenesis techniques or by direct synthesis. The term "conservative amino acid substitution" refers to a substitution of a native amino acid residue with a normative residue such that there is little or no effect on the polarity or charge of the amino acid residue at that position. For example, a conservative substitution results from the replacement of a non-polar residue in a polypeptide with any other non-polar residue. Another example of a conservative substitution is the replacement of an acidic residue with another acidic residue. Conservative substitutions are expected to produce ABCl polypeptides having functional and chemical characteristics similar to those of the naturally-occurring ABCl polypeptide. The term "ortholog" refers to a polypeptide that corresponds to a polypeptide identified from a different species. For example, mouse and human ABCl polypeptides are considered orthologs. The term "vector" is used to refer to any molecule (e.g. nucleic acid, plasmid, or virus) used to transfer coding informationto a host cell. The term "expression vector" refers to a vector that is suitable for transformation of a host cell and contains nucleic acid sequences that direct and/or control the expression of inserted heterologous nucleic acid sequences. Expression includes, but is not limited to, processes such as transcription, translation, and RNA splicing, if introns are present. As used herein, the term "transcriptional regulatory region" or "expression modulating portion" refers to any region of the gene, including, but not limited to, promoters, enhancers, and repressors. As used herein, the term "promoter" refers to an untranscribed sequence located upstream (i.e., 5') to the start codon of a structural gene (generally within about 100 to 1000 bp) that controls the transcription of the structural gene. As used herein the term "enhancers" refers to cis-acting elements of DNA, usually about 10-300 bp in length, that act on the promoter to increase transcription. Enhancers are relatively orientation and position independent. They have been found 5' and 3' to the transcription unit. "Host cell" is a cell that has been transformed or transfected, or is capable of transformation or transfection by an exogenous polynucleotide sequence. The term includes the progeny of the parent cell, whether or not the progeny is identical in morphology or in genetic make-up to the original parent, so long as the selected gene is present. The term "operatively linked" is used herein to refer to an arrangement of flanking sequences wherein the flanking sequences so described are configured or assembled so as to perform their usual function. Thus, a flanking sequence operably linked to a coding sequence may be capable of effecting the replication, transcription and/or translation of the coding sequence. For example, a coding sequence is operably linked to a promoter when the promoter is capable of directing transcription of that coding sequence. A flanking sequence need not be contiguous with the coding sequence, so long as it functions correctly. Thus, for example, intervening untranslated yet transcribed sequences can be present between a promoter sequence and the coding sequence and the promoter sequence can still be considered "operably linked" to the coding sequence. The term "transfection" is used to refer to the uptake of foreign or exogenous DNA by a cell, and a cell has been "transfected" when the exogenous DNA has been introduced inside the cell membrane. A number of transfection techniques are well known in the art and are disclosed herein. See, e.g., Graham et al., 1973, Virology 52:456: Sambrook et al., Molecular Cloning, A Laboratoiy Manual (Cold Spring Harbor Laboratories, 1989); Davis et al, Basic Methods in Molecular Biology (Elsevier, 1986); and Chu et al, 1981 , Gene 13: 197. Such techniques can be used to introduce one or more exogenous DNA moieties into suitable host cells. ABCl Polypeptides The present invention relates to novel human ABCl polypeptides. In one embodiment, the ABCl polypeptide comprises the amino acid sequence shown in SEQ ID NO: 2. In contrast to the human ABCl protein reported by others, the ABCl polypeptide shown in SEQ ID NO: 2 has an additional 60 amino acids at the amino terminus, revealing an ABCl protein of 2261 amino acids rather than 2201 amino acids (see Langmann et al. in Biochem. Biophys. Res. Comm., 257, 29-33 (1999)). In addition, the ABCl polypeptide shown in SEQ ID NO: 2 differs from other reported sequences at several amino acid residues. Specifically, the present ABCl polypeptide shown in SEQ ID NO: 2 has a K for R at residue 159, 1 for V at 765, M for I at 823, I for T at 1495, L for P at 1588, K for R at 1914, and L for P at 2108. To remain consistent with published notation, the above amino acid numbers are those of Lawn et al., J. Clin. Invest., 104: R25-31 (1999), rather than those of SEQ ID NO: 2. As discussed in further detail below, the sequence difference likely arises from the fact that the first ABCl cDNA was cloned from mouse using a PCR-based strategy and the subsequently reported human ABCl cDNA sequences were predicted from the sequence of the mouse protein. The ABCl protein has an approximate molecular weight of 240 kD as determined by SDS-PAGE. The present invention also relates to ABCl polypeptides comprising amino acid sequences which preferably have at least 98% identity over their entire length to the amino acid sequence of SEQ ID NO: 2. More preferably, the polypeptide has at least 99% identity over its entire length to the amino acid sequence of SEQ ID NO: 2. Most preferably, the polypeptide has 100% identity over its entire length to the amino acid sequence of SEQ ID NO: 2. As defined previously, the term "identity"refers to the degree of sequence relatedness between polypeptide sequences, which is further defined below. Such related ABCl polypeptides include substitution, deletion, and insertion variants, as well as allelic variants, splice variants, fragments, derivatives, and orthologs. Preferred polypeptides and polypeptides fragments include those polypeptides and fragments that possess the biological activity of ABCl. In particular, those polypeptides and fragments that mediate reverse cholesterol transport are preferred. Also preferred are polypeptides and fragments that have improved reverse cholesterol transport activity. Substitution, deletion, and insertion variants refer to ABCl polypeptides comprising amino acid sequences that contain one or more amino acid sequence substitutions, deletions, and or additions as compared to the ABCl amino acid sequence set forth in SEQ ID NO: 2. In preferred embodiments, the variants have from about 1 to 5, or from about 1 to 10, or from about 1 to 20, or from about 1 to 40, or from about 1 to 65 amino acid substitutions, additions, and/or deletions. For example, the variants can have an addition of one or more amino acid residues anywhere in the polypeptide as well as at the carboxyl terminus and/or at the amino terminus, as long as the variant retains biological function. Also, for example, one or more amino acids can be deleted from any region of the polypeptide, including the carboxyl terminus and/or amino terminus, without substantial loss of biological function (Ron et al., J. Biol. Chem., 268: 2984- 2988 (1993); Dobeli et al, J. Biotechnology, 7: 199-216 (1988)). The amino acid substitution(s) can be conservative, non-conservative, or any combination thereof, as long as the ABCl variant retains its biological activity. In addition, the substitution(s) can be with non-conserved amino acid residues, where the substituted residues may or may not be encoded by the genetic code, and with amino acid residues having a substituent group. Suitable variants of ABCl polypeptides can be determined using well-known techniques. For example, suitable ABCl variants can be determined by identifying regions of the ABCl molecule that may be changed without destroying biological activity. Also, as realized in the art, even regions that may be important for biological activity or for structure may be subject to conservative amino acid substitutions without destroying the biological activity or without adversely affecting the polypeptide structure. Amino acid residues that can be changed without destroying biological activity can be determined by identifying regions of the ABC 1 polypeptide that are not important for activity (Bowie et al., Science, 247: 1306-1310 (1990)). For example, ABCl polypeptides from various species can be compared to determine the amino acid residues and regions of ABCl molecules that are conserved across species. The conserved amino acid residues are likely important for biological function and/or structure. In contrast, changes in regions of the ABCl molecule that are not conserved across species and are thus tolerated by natural selection would be less likely to adversely affect biological activity and/or structure. Accordingly, ABCl polypeptides with additions, deletions, or substitutions in the non-conserved regions are likely suitable variants. Even in relatively conserved regions, chemically similar amino acids may be substituted for the naturally occurring residues while retaining activity. In addition, suitable ABCl variants can be identified using structure-function studies to determine residues in other members of the ATP -binding cassette protein family, such as ABCR and ABC-C, that are important for activity or structure. Such studies allow the prediction of important amino acid residues in an ABC 1 variant that correspond to amino acid residues that are important for activity or structure in other ATP-binding cassette proteins. For example, based on structure-function studies of other ATP-binding cassette proteins, important amino acid residues in ABCl are likely found in regions associated with nucleotide binding and sterol transport. Suitable variants include, for example, polypeptides having chemically similar amino acid substitutions for such predicted important amino acid residues of the ABCl polypeptide. Suitable ABCl variants can also be determined using genetic engineering techniques to introduce amino acid changes at specific positions in order to identify regions critical for polypeptide function. Amino acid changes can be made using, for example, site-directed mutagenesis or alanine-scanning mutagenesis (Cunningham et al., Science, 244: 1081-1085 (1989)). The resultant ABCl variants can then be tested for biological activity using, for example, any one of the cholesterol efflux assays described herein. Variants having a particular amino acid residue substituion that results in destroyed cholesterol efflux activity would not be considered a suitable ABC 1 variant. Additional methods for identifying suitable variants are known in the art. Furthermore, one skilled in the art would realize amino acid changes that are likely to be permissive at certain amino acid positions in the protein (Bowie et al., supra (1990)). For example, it is generally known that the most buried or interior (within the tertiary structure of the protein) amino acid residues require nonpolar side chains, whereas fewer features of surface or exterior side chains are generally conserved. Moreover, it is known that tolerated conservative amino acid substitutions involve replacement of the aliphatic or hydrophobic amino acids Ala, Val, Leu and He, replacement of the hydroxyl residues Ser and Thr, replacement of the acidic residues Asp and Glu, replacement of the amide residues Asn and Gin, replacement of the basic residues Lys, Arg, and His, replacement of the aromatic residues Phe, Tyr, and Tip, and replacement of the small- sized amino acids Ala, Ser, Thr, Met, and Gly. The ABC 1 variants can be naturally-occurring or artificially constructed. Examples of naturally-occurring variants are allelic variants and splice variants. Allelic variants refer to one of several alternate forms of a gene occupying a given locus on a chromosome of an organism or population of organisms (Lewin, B., ed., Genes II, John Wiley & Sons, New York (1985)). Allelic variants can vary at either the polynucleotide and/or polypeptide level. Splice variants refer to a nucleic acid molecule, usually RNA, which is generated by alternative processing of intron sequences in an RNA transcript, and the corresponding polypeptide. Alternatively, the ABCl variants can be artificially constructed. For example, ABCl variants can be constructed using the technique of site-directed mutagenesis. Also, for example, ABCl variants can be prepared from the corresponding nucleic acid molecules encoding said variants, which have a DNA sequence that varies as described from the wild-type DNA sequence as set forth in SEQ ID NO: 1. Polypeptide fragments refer to polypeptides which comprise less than the full length amino acid sequence of ABCl set forth in SEQ ID NO: 2. Preferred polypeptides fragments include those fragments that possess the biological activity of ABCl. In particular, those fragments that mediate reverse cholesterol transport or have improved reverse cholesterol transport activity are preferred. ABCl fragments can have one or more amino acids deleted from any region of the polypeptide, including the carboxyl terminus and/or amino terminus, as long as biological function is maintained. The ABCl polypeptide fragments can occur naturally, such through alternative splicing or in vivo protease activity, or can be artificially constructed using well-known methods. The invention also relates to ABCl polypeptide derivatives, which refer to ABCl polypeptides, variants, or fragments, as defined herein, that have been chemically modified. The derivatives are modified in a manner that is different from naturally-occurring ABCl polypeptides, either in type or location of the molecules attached to the polypeptide. Derivatives may further include polypeptides formed by the deletion of one or more chemical groups which are naturally attached to the ABCl polypeptides. In addition, the ABCl polypeptide comprising the amino acid sequence of SEQ ID NO: 2, as well as the above-described ABCl variants and fragments, may be fused to a homologous polypeptide to form a homodimer or to a heterologous polypeptide to form a heterodimer. Another aspect of the present invention relates to mutant ABC 1 polypeptides and fragments thereof, corresponding to polypeptides isolated from Tangier patients. In one preferred embodiment, the ABCl polypeptide comprises SEQ ID NO: 8. The protein was isolated from a Tangier patient (TD1) and sequenced as decribed in Examples 1 and 5. The amino acid sequence set forth in SEQ ID NO: 8 is similar to the wild-type sequence with the exception of a glutamine to arginine residue substitution at position 537 (the residue number is that of Lawn et al, J. Clin. Invest., 104: r25-31 (1999), corresponding to position 597 of SEQ ID NO: 8). The location of this residue is within the amino-terminal hydrophilic domain, near the first predicted transmembrane domain. The substitution alters the charge of the amino acid in this region of the protein, resulting in an ABCl protein that has significantly decreased cholesterol efflux activity, as shown in Figure 1. In another preferred embodiment, the mutant ABCl polypeptide comprises SEQ ID NO: 10. The protein corresponds to a polypeptide isolated from a Tangier patient (TD2) and sequenced as decribed in Examples 1 and 5. The amino acid sequence set forth in SEQ ID NO: 10 is similar to the wild-type sequence with the exception of an arginine to tryptophan substitution at residue 527 (the residue number is that of Lawn et al., supra (1999), corresponding to position 587 of SEQ ID NO: 10). Like the TD1 polypeptide, this substitution alters the charge of the amino acid residue in the amino-terminal hydrophilic domain of the ABCl protein. The resultant mutant ABCl protein also has significantly decreased cholesterol efflux activity, as shown in Figure 1. ABCl Polynucleotides Another aspect of the present invention relates to isolated polynucleotides that encode the novel ABCl polypeptides and variants thereof. The present invention provides, for example, isolated polynucleotides encoding the full-length ABCl polypeptide, polynucleotides containing the full-lenth cDNA of wild-type ABC 1 , polynucleotides containing the entire length of the coding sequence of wild-type ABCl, and polynucleotides containing non-coding 5' and 3' sequences of ABCl, as well as polynucleotides of related ABCl variants. The present invention also provides isolated polynucleotides that encode mutant ABCl polypeptides, such as those of Tangier patients. In one preferred embodiment, the isolated polynucleotide comprises a nucleotide sequence encoding the polypeptide comprising SEQ ID NO: 2. Importantly, in contrast to the published sequence of Langmann et al. which codes for a protein of 2201 amino acids based on a predicted start methionine found in exon 3 (Langmann et al., Biochem. Biophys. Res. Comm., 257: 29-33 (1999) (GenBank Accession No. AJO 12376), the presently claimed nucleotide sequence contains 50 exons and codes for a protein of 2261 amino acids (see Figure 4). The corresponding nucleotide sequence of the present invention contains a coding sequence that includes an additional 180 nucleotides at the 5' end corresponding to the following 60 amino- terminal amino acids: MACWPQLRLLLWKNLTFRRRQTCQLLLEVAWPLFIFLILISVRLSYPPYEQHECHFPNKA . Given that there is an in-frame stop codon 6 to 9 nucleotides upstream from this location, the newly predicted start site is the first methionine codon that could produce a continuous open reading frame. Alignment of this new ABCl cDNA sequence with related ABC transporter sequences ABCR and ABC-C (also known as ABC3) which also contain open reading frames for the 60 additional amino acids, indicates a high degree of similarity, implying that the homologous ABC transporter proteins begin with sequences related to the amino terminal extension sequence proposed for human ABC 1. It is likely that the earlier published start site of the human ABCl was predicted from the published mouse ABCl cDNA sequence (Luciani et al, Genomics, 21 150-159 (1994); GenBank Accession no.: X75926) which contains an extra nucleotide "n" in the extension region such that the newly disclosed methionine is not in- frame. However, if the "n" nucleotide in the mouse sequence is ignored, the mouse and human sequences of the extension region are identical. In light of these results, it is likely that the full length human ABCl protein contains 2261 amino acids rather than 2201 amino acids, as previously suggested by Langmann et al. and others. Accordingly, Langmann et al. do not present the full open reading frame of human ABCl . In another preferred embodiment, the isolated polynucleotide comprises the full-lenth ABCl cDNA, including at least a portion of either non-coding 5' and 3' sequences. Preferably, the polynucleotide comprises the nucleotide sequence shown in SEQ ID NO: 1. The 10.4 kb human ABCl cDNA sequence shown in SEQ ID NO: 1 contains an open reading frame of 6783 nucleotides plus 5' and 3' untranslated regions. There is a start codon at position 291 and a stop codon at position 7074. The present ABCl cDNA shown in SEQ ID NO: 1 differs from the published ABCl cDNA (GenBank Accession No. AJO12376) in several respects. First, the present ABCl cDNA includes an additional 350 nucleotides at the 5' end and an additional 3136 nucleotides at the 3' end (not including the poly(A) tail). The present ABCl sequence also differs from the published ABCl cDNA by the substitution of 10 nucleotides in the coding region. Of the ten differences, seven nucleotide differences predict amino acid changes. To remain consistent with published notation, the following nucleotide and amino acid numbers are those of Lawn et al., J. Clin. Invest., 104: R25-31 (1999) and GenBank Accession No. AJO12376, rather than those of SEQ ID NO: 1. The nucleotide and amino acid changes are as follows: (1) A for G at nucleotide 414; (2) A for G at nucleotide 596 (K for R at amino acid 159); (3) T for C at nucleotide 705; (4) A for C at nucleotide 1980; (5) A for G at 2413 (I for V at amino acid 765); (6) G for A at 2589 (M for I at amino acid 823); (7) T for C at 4604 (I for T at amino acid 1495); (8) T for C at 4883 (L for P at amino acid 1588); (9) A for G at 5861 (K for R at amino acid 1914); and ( 10) T for C at 6443 (L for P at amino acid 2108). Five of the amino acid changes are conservative amino acid changes and may represent polymorphisms or sequence errors. In two instances, the present sequence predicts important amino acid differences from the GenBank sequence. The differences result in a leucine rather than a proline at residue 1588 and at residue 2108. Interestingly, at both positions, the predicted leucine was also found in each of the three TD samples analyzed as well as the highly conserved mouse ABCl protein sequence. The present invention also relates to ABCl polynucleotides comprising nucleotide sequences that preferably have at least 80% identity over their entire length to the polynucleotide comprising SEQ ID NO: 1. More preferably, the polynucleotide has at least 90% identity over its entire length to the polynucleotide comprising SEQ ID NO: 1. Even more preferably, the polynucleotide has at least 95% identity over its entire length to the polynucleotide comprising SEQ ID NO: 1. Most preferably the polynucleotide has 100% identity over its entire length to the polynucleotide comprising SEQ ID NO: 1. Such related ABCl polynucleotides include substitution, deletion, and insertion variants, as well as allelic variants, splice variants, fragments, derivatives, and orthologs, wherein one or more nucleotides have been substituted, deleted, inserted, or derivatized. Preferred polynucleotides include those polynucleotides that encode ABCl polypeptides possessing biological activity, such as cholesterol efflux activity. In another preferred embodiment, the isolated polynucleotide comprises the entire coding sequence of ABCl . In a particularly preferred embodiment, the polynucleotide comprises the sequence shown as nucleotides 291-7074 of SEQ ID NO: 1. This isolated polynucleotide contains an ABCl open reading frame of 6783 nucleotides and encodes a polypeptide of 2261 amino acids, as described above. In yet another preferred embodiment, the isolated polynucleotide comprises a nucleotide sequence that encodes an ABC 1 variant polypeptide. In particular, the isolated polynucleotide comprises a nucleotide sequence that encodes a polypeptide comprising an amino acid sequence which is at least 98% identical to the amino acid sequence of SEQ ID NO: 2. Also preferred is an isolated polynucleotide comprising a nucleotide sequence that encodes a polypeptide comprising an amino acid sequence which is at least 99% identical to the amino acid sequence of SEQ ID NO: 2. Accordingly, the invention includes those polynucleotides that encode the above-described ABCl polypeptides, including the described substitution, deletion, and insertion variants, as well as ABCl allelic variants, splice variants, fragments, derivatives, fusion polypeptides, and orthologs. Preferred polynucleotides are those polynucleotides that encode polypeptides that possess the biological activity of ABCl. In particular, those polynucleotides that encode polypeptides that mediate reverse cholesterol transport are preferred. Also preferred are polynucleotides that enocode polypeptides that have improved reverse cholesterol transport activity. Yet another aspect of the invention relates to isolated polynucleotides that encode mutant ABCl polypeptides from Tangier patients. In one preferred embodiment, the polynucleotide encodes the polypeptide of SEQ ID NO: 8, which polypeptide is isolated from patient TD1 and is described above. In another preferred embodiment, the polynucleotide comprises the nucleotide sequence set forth in SEQ ID NO: 7. The nucleotide sequence set forth in SEQ ID NO: 7 contains the full open reading frame, as well as 5' and 3' flanking sequences. The open reading frame encodes a polypeptide of 2261 amino acids, containing, among other substitutions, a nucleotide substitution that results in an A to G substitution at position 537 (using the numbering of Lawn et al., supra (1999)). In another preferred embodiment relating to polynucleotides that encode mutant ABCl polypeptides, the polynucleotide encodes the polypeptide of SEQ ID NO: 10, which polypeptide is isolated from Tangier patient TD2 and is also described above. In yet another preferred embodiment, the polynucleotide comprises the nucleotide sequence set forth in SEQ ID NO: 9. The nucleotide sequence set forth in SEQ ID NO: 9 contains the full open reading frame, as well as 5' and 3' flanking sequences. The open reading frame encodes a polypeptide of 2261 amino acids, containing, among other substitutions, a polynucleotide substitution that results in an Arg to Tryp substitution at residue 527 (using the numbering of Lawn et al., supra (1999)). Another aspect of the present invention relates to isolated polynucleotides that comprise the non-coding 5' flanking and 3' flanking regions of ABCl . In one embodiment, the isolated polynucleotide comprises the non-coding 5' flanking region of ABCl. Preferably, the 5' flanking region contains, but is not limited to, the ABCl promoter region. Thus, in a preferred embodiment, the polynucleotide comprises the sequence shown in SEQ ID NO: 3. As demonstrated by heterologous reporter assays, discussed in Example 15, the polynucleotide set forth in SEQ ID NO: 3 contains the transcriptional regulatory region of the ABCl gene. As shown in Figure 13, the polynucleotide set forth in SEQ ID NO: 3 is a 1643 b.p. non-coding sequence that contains several transcription regulatory elements, including a TATA box at positions 1522, 1435, and 1383, as well as transcription factor binding sites, including several putative SP 1 sites, and several nuclear receptor half sites. In addition, an identified sterol response element is found at position 1483-1500. Further heterologous reporter assays described in Example 17 revealed that several discrete portions of SEQ ID NO: 3 retained promoter activity. Accordingly, in another preferred embodiment, the polynucleotide comprises nucleotides 1-1532, 1080-1643, 1 181-1643, 1292-1643, or 1394-1643 of SEQ ID NO: 3. In an especially preferred embodiment, the polynucleotide comprises nucleotides 1394-1532 of SEQ ID NO: 3, which sequence has been shown to have ABC l promoter activity (see Example 17). In yet another preferred embodiment, the polynucleotide comprises nucleotides 1480-1510 of SEQ ID NO: 3, which is shown to regulate the ABCl transcriptional response to LXR ligands. The 5' flanking polynucleotide also comprises a nucleotide sequence that hybridizes, under stringent conditions, to the nucleotide sequence set forth in SEQ ID NO: 3, wherein the nucleotide sequence has ABCl promoter activity. In yet another embodiment, the polynucleotide comprises a nucleotide sequence that hybridizes, under stringent conditions, to the nucleotide sequence comprising nucleotides 1-1532, 1080-1643, 1 181-1643, 1292-1643, or 1394-1643 of SEQ ID NO: 3, wherein the nucleotide sequence has ABCl promoter activity. In another embodiment, the isolated polynucleotide comprises the 3' flanking region of ABCl . Several 3' untranslated regions have been identified which may represent alternate sites of polyadenylation of the ABCl transcript. Preferably, the 3' flanking region contains regulatory sequences. For example, the full length 3' UTR (SEQ ID NO: 6) contains 46 sequences (AA)nCU UC(AA)n which have been shown to be necessary for binding of Vigilin. Vigilin, a ubiquitous protein with 14K homology domains, is the estrogen-inducible vitellogenin mRNA 3 '-untranslated region binding protein (J. Biol. Chem., 272: 12249-12252 (1997)). In addition to binding HDL, Vigilin has been shown to bind to the 3' flanking region of mRNAs and to increase the half-life of the mRNA transcript (Mol. Cell Biol, 18:3991-4003 (1998)). Thus, the 3' flanking region could be altered, for example, to increase the binding of Vigilin, thereby increasing the half-life of the ABCl mRNA. Preferably, the isolated polynucleotide comprises the sequence shown in SEQ ID NO: 4. Also preferably, the isolated polynucleotide comprises the sequence shown in SEQ ID NO: 5. In another preferred embodiment, the isolated polynucleotide comprises the sequence shown in SEQ ID NO: 6. In other preferred embodiments, the polynucleotide comprises a sequence that hybridizes, under stringent conditions, to the nucleotide sequence set forth in SEQ ID NO: 4, SEQ ID NO: 5, or SEQ ID NO: 6. The present invention also includes related ABCl 5' and 3' flanking polynucleotides. Accordingly, the invention relates to polynucleotides comprising nucleotide sequences that have at least 80% identity over their entire length to the polynucleotide comprising SEQ ID NO: 3, the polynucleotide comprising SEQ ID NO: 4, the polynucleotide comprising SEQ ID NO: 5, the polynucleotide comprising SEQ ID NO: 6, or the polynucleotide comprising nucleotides 1- 1532, 1080-1643, 1 181-1643, 1292-1643, or 1394- 1643 of SEQ ID NO: 3. Preferably, the polynucleotide has at least 80%, more preferably at least 90%, even more preferably at least 95%, and most preferably 100% identity over its entire length to any one of the aforementioned flanking polynucleotides. Preferred polynucleotides include those polynucleotides that possess biological activity, such as transcriptional regulatory activity. It is understood that the present invention further relates to isolated polynucleotides that are complementary to any one of the above-described polynucleotide sequences. As used herein, the term "complementary" refers to the hybridization or base pairing between nucleotides, such as, for example, between the two strands of a double-stranded polynucleotide. Two single-stranded nucleotide molecules are said to be complementary when the nucleotides of one strand, optimally aligned with appropriate nucleotide insertions, deletions or substitutions, pair with at least about 80% of the nucleotides of the other strand. Another aspect of the present invention relates to compositions comprising the novel ABC 1 polynucleotides described above and a suitable carrier. In one embodiment, the composition comprises a polynucleotide encoding the polypeptide comprising SEQ ID NO: 2, a polynucleotide comprising SEQ ID NO: 1, a polynucleotide comprising nucleotides 291-7074 of SEQ ID NO: 1, or a polynucleotide encoding a polypeptide comprising an amino acid sequence which is at least 98% identical to the amino acid sequence of SEQ ID NO: 2 and a suitable carrier. In another embodiment, the composition comprises a polynucleotide having at least 80%, preferably 90%, or more preferably 95% identity over its entire length to the polynucleotide comprising SEQ ID NO: 1 and a suitable carrier. In another embodiment, the composition comprises a polynucleotide comprising an ABCl 5' flanking sequence and a suitable carrier. Preferably, the composition comprises a polynucleotide comprising SEQ ID NO: 3 or a polynucleotide comprising nucleotide fragments 1-1532, 1080-1643, 1 181-1643, 1292-1643, or 1394-1643 of SEQ ID NO: 3 and a suitable carrier. Also preferably, the composition comprises a polynucleotide having at least 80%, 90%, or 95% identity over its entire length to the polynucleotide comprising any one of the described 5' flanking sequences and a suitable carrier. In yet another embodiment, the composition comprises a polynucleotide comprising an ABCl 3' flanking sequence and a suitable carrier. Preferred compositions comprise a polynucleotide comprising SEQ ID NO: 4, a polynucleotide comprising SEQ ID NO: 5, or a polynucleotide comprising SEQ ID NO: 6, as well as a polynucleotide having at least 80%, preferably 90%, or more preferably 95% identity over its entire length to any of these polynucleotides, and a suitable carrier. Still other compositions of the invention comprise mutant ABCl polynucleotides. Preferably, the composition comprises a polynucleotide comprising SEQ ID NO: 7 or a polynucleotide comprising SEQ ID NO: 9 and a suitable carrier. In addition, a composition of the present invention may comprise, in any combination, two or more of the above-described ABCl polynucleotides and a suitable carrier. Any suitable aqueous carrier can be used in the composition. Preferably, the carrier renders the composition stable at a desired temperature, such as room temperature or storage temperature (i.e. 4°C to - 20°C), and is of approximately neutral pH. Examples of suitable carriers are known to those of skill in the art and include Tris-EDTA buffer and DEPC-H 2 O. ABCl Vectors and Host Cells The present invention also relates to recombinant vectors that comprise one or more of the above-described ABCl polynucleotides, host cells that are genetically engineered with the vectors comprising ABCl polynucleotides, and the production of ABCl polypeptides by recombinant techniques. As mentioned, the invention provides recombinant vectors that comprise one or more of the above-described wild-type ABCl polynucleotides. In preferred embodiments, the recombinant vector comprises the polynucleotide encoding the polypeptide comprising SEQ ID NO: 2, the polynucleotide comprising SEQ ID NO: 1, and the polynucleotide comprising nucleotides 291-7074 of SEQ ID NO: 1. In another preferred embodiment, the recombinant vector comprises the variant polynucleotide encoding a polypeptide comprising an amino acid sequence which is at least 98% identical to the amino acid sequence of SEQ ID NO: 2. Also, in another preferred embodiment, the recombinant vector comprises a variant polynucleotide that is at least 80% identical to a polynucleotide comprising SEQ ID NO: 1. In still another preferred embodiment, the recombinant vector comprises a polynucleotide that is complementary to any of these polynucleotides. In another embodiment, the recombinant vector comprises a polynucleotide comprising a mutant ABCl polynucleotide isolated from a Tangier disease patient. In a preferred embodiment, the recombinant vector comprises the polynucleotide comprising SEQ ID NO: 8. In another preferred embodiment, the recombinant vector comprises the polynucleotide comprising SEQ ID NO: 10. The recombinant vectors may also comprise a polynucleotide sequence that is complementary to these sequences. It is also understood that the recombinant vector may also comprise, in any combination, two or more of the above-described wild-type, variant, or mutant ABCl polynulceotides. An isolated ABC l polynucleotide, such as any of the above-described wild-type, variant, or mutant polynucleotides, is inserted into a vector using well-known ligation and cloning techniques. Cloning techniques have been described in several standard laboratory manuals, including Davis et al., Basic Methods in Molecular Biology (1986); Sambrook et al., Molecular Cloning: A Laboratory Manual, 2 nd Ed., (Cold Spring Harbor Press, Cold Spring Harbor, N.Y. (1989)); Ausubel et al. eds., Current Protocols in Molecular Biology, (Wiley and Sons (1994)); Goeddel, ed., Gene Expression Technology (Methods in Enzymology ( 1991)); Murray, ed., Gene Transfer and Expression Protocols (Human Press, Clifton, NJ). Any vector suitable for ABC 1 polynucleotide insertion can be used. The vector is typically selected to be functional in the particular host cell employed (i.e., the vector is compatible with the host cell machinery such that amplification and/or expression of the gene can occur). Preferably, the vector is compatible with bacterial, insect, or mammalian host cells. Also preferably, the vector is an expression vector (for a review of expression vectors, see Goeddel, D.V. ed., Methods Enzymol, Academic Press Inc., San Diego, CA (1990)). The vector may be, for example, a phage, plasmid, viral, or retroviral vector. Retro viral vectors may be replication competent or replication defective. In the latter case, viral propagation generally will occur only in complementing host cells. Typically, expression vectors used in any of the host cells will contain sequences for plasmid maintenance and for cloning and expression of exogenous nucleotide sequences. Such sequences, collectively refeoed to as "flanking sequences" should preferably include one or more of the following nucleotide sequences: a promoter, one or more enhancer sequences, an origin of replication, a transcriptional termination sequence, a complete intron sequence containing a donor and acceptor splice site, a sequence encoding a leader sequence for polypeptide secretion, a ribosome binding site, a polyadenylation sequence, a polylinker region for inserting the nucleic acid encoding the polypeptide to be expressed, and a selectable marker element. The flanking sequences may be homologous (i.e., from the same species and/or strain as the host cell), heterologous (i.e., from a species other than the host cell species or strain), hybrid (i.e., a combination of flanking sequences from more than one source), or synthetic. Also, the flanking sequences may be native sequences which normally function to regulate ABCl polypeptide expression. The source of a flanking sequence may be any prokaryotic or eukaryotic organism, any vertebrate or invertebrate organism, or any plant, provided that the flanking sequence is functional in, and can be activated by, the host cell machinery. The vector should also preferably include at least one selectable marker for propagation in a host. A selectable marker is a gene element that encodes a protein necessary for the survival and growth of a host cell grown in a selective culture medium. Suitable selection marker genes encode proteins that (a) confer resistance to antibiotics or other toxins, e.g. , ampicillin, tetracycline, or kanamycin for prokaryotic host cells; (b) complement auxotrophic deficiencies of the cell; or (c) supply critical nutrients not available from complex media. Prefeoed selectable markers include zeocin, G418, hygromycin, or neomycin resistence for eukaryotic cell culture and tetracycline, kanamycin, or ampicillin resistance for culturing in E. coli and other bacteria. Other suitable selection genes include those that are used to amplify the expressed gene. Amplification is the process wherein genes that are in greater demand for the production of a protein critical for growth are reiterated in tandem within the chromosomes of successive generations of recombinant cells. Examples of suitable selectable markers for mammalian cells include dihydrofolate reductase (DHFR) and thymidine kinase. The mammalian cell transformants are placed under selection pressure wherein only the transformants are uniquely adapted to survive by virtue of the selection gene present in the vector. Selection pressure is imposed by culturing the transformed cells under conditions in which the concentration of selection agent in the medium is successively changed, thereby leading to the amplification of both the selection gene and the ABCl gene. As a result, increased quantities of ABCl polypeptide are synthesized from the amplified DNA. The vector should also preferably contain a transcription termination sequence, which is typically located 3' of the end of a polypeptide coding region and serves to terminate transcription. Usually, the transcription termination sequence in prokaryote cells is a G-C rich fragment followed by a poly T sequence. The sequence can be purchased as part of a commercial vector or synthesized using well-known methods for nucleic acid synthesis. The vector should also preferably contain a ribosome binding site, which is usually necessary for translation initiation of mRNA and is characterized by a Shine-Dai garno sequence (prokaryotes) or a Kozak sequence (eukaryotes). The element is typically located 3' to the promoter and 5' to the coding sequence of the ABCl polypeptide to be expressed. The Shine- Dalgaoio sequence is varied but is typically a polypurine (i.e., having a high A-G content). Many Shine-Dalgarno sequences have been identified, each of which can be readily synthesized using well-known methods. The vector should also preferably contain a promoter that is recognized by the host organism and operably linked to the encoded polynucleotide. The promoter can be an inducible promoter or a constitutive promoter. Inducible promoters initiate increased levels of transcription from DNA under their control in response to some change in culture conditions, such as the presence or absence of a nutrient or a change in temperature. In contrast, constitutive promoters initiate continual gene product production; consequently there is little or no control over gene expression. A suitable promoter is operably linked to a polynucleotide, by removing the promoter from the source DNA by restriction enzyme digestion and inserting the desired promoter sequence into the vector. As mentioned, a native promoter can be used to direct amplification and/or expression of the polynucleotide. Thus, the recombinant vector can comprise any one of the above-described wild-type, variant, or mutant ABCl polynulceotides and an ABCl promoter, such as that found in SEQ ID NO: 3. The recombinant vector can also comprise, in any combination, two or more of the above-described wild-type, variant, or mutant ABCl polynulceotides and an ABCl promoter. Preferably, a heterologous promoter is used if it permits greater transcription and higher yields of ABCl protein as compared to the native ABCl promoter, and if it is compatible with the host cell system that has been selected for use. The heterologous promoter can be used alone or in conjunction with the native ABC 1 promoter. Thus, in one preferred embodiment, the recombinant vector comprises any one of the above-described wild-type ABCl polynucleotides and a heterologous promoter. In another preferred embodiment, the recombinant vector comprises any one of the above-described variant ABCl polynucleotides and a heterologous promoter. In yet another prefeoed embodiment, the recombinant vector comprises any one of the above-described mutant ABCl polynucleotides and a heterologous promoter. Prefeoed embodiments also include recombinant vectors that contain any combination of two or more of the above-described wild-type, variant, and mutant ABCl polynucleotides and a heterologous promoter. Heterologous promoters suitable for use with prokaryotic hosts include, but are not limited to, the beta-lactamase and lactose promoter systems (Villa-Kamaroff et al, 1978, Proc. Natl Acad. Sci. U.S.A., 75:3727-31), alkaline phosphatase, a tryptophan (tip) promoter system, and hybrid promoters such as the tac promoter (Villa-Kamaroff et al, 1978, Proc. Natl. Acad. Sci. U.S.A., 75:3727-31). Their sequences have been published, thereby enabling one skilled in the art to ligate them to the desired DNA sequence, using linkers or adapters as needed to supply any useful restriction sites. Other suitable heterologous promoters will be known to those skilled in the art. Suitable heterologous promoters for use with mammalian host cells are also well known and include, but are not limited to, those obtained from the genomes of viruses such as polyoma virus, fowlpox virus, adenovirus (such as Adenovirus 2), bovine papilloma virus, avian sarcoma virus, cytomegalovirus, retroviruses, hepatitis-B virus and Simian Virus 40 (SV40). Other suitable mammalian promoters include heterologous mammalian promoters, for example, heat- shock promoters and the actin promoter. Additional suitable promoters include, but are not limited to: the SV40 early promoter and late promoter region (Bernoist and Chambon, 1981, Nature 290:304-10); the promoter contained in the 3' long terminal repeat of Rous sarcoma virus (Yamamoto, et al, 1980, Cell 22:787-97); the herpes thymidine kinase promoter (Wagner et al, 1981, Proc. Natl. Acad. Sci. U.S.A. 78: 1444-45); and the regulatory sequences of the metallothionine gene (Brinster et al, 1982, Nature 296:39-42). Preferably, the promoter is a cytomegalovirus or SV40 promoter. Thus, in especially prefeoed embodiments, the recombinant vector comprises one of the above-described wild-type ABCl polynucleotides, one of the above- described variant ABCl polynucleotides, or one of the above-described mutant ABCl polynucleotides and a cytomegalovirus promoter. In another especially prefeoed embodiment, the recombinant vector comprises, in any combination, two or more of the above-described wild- type, variant, or mutant ABCl polynulceotides and a cytomegalovirus promoter. The vector also preferably contains an enhancer sequence to increase the transcription of a polynucleotide, such as ABCl . Suitable enhancers for the activation of eukaryotic promoters include viral enhancers, such as the SV40, cytomegalovirus early promoter, polyoma, and adenovirus enhancers. Expression vectors of the invention may be constructed from a starting vector, such as a commercially available vector, which may or may not contain all of the desired flanking sequences. Where one or more of the flanking sequences described herein are not already present in the vector, they may be individually obtained and ligated into the vector. Methods used for obtaining each of the flanking sequences are well known to one skilled in the art. Prefeoed vectors are those which are compatible with bacterial, insect, and mammalian host cells. Vectors prefeoed for use in bacteria include, for example, pQE70, pQE60, and pQE-9 (Quiagen, Inc.), pBluescript vectors, Phagescript vectors, pNH16A, pNH18A, pNH46A (Stratagene Cloning Systems, Inc.), ptrc99a, pKK223-3, pDR540, pRIT5 (Pharmacia Biotech, Inc.), and pCEP4 (Invitrogen Corp., Carlsbad, CA). Prefeoed eukaryotic vectors include, but are not limited to, pWLNEO, pSV2CAT, pOG44, pXTI and pSG (Stratagene), pSVK3, pBPV, pMSG, and pSVL (Pharmacia), and pGL3 (Promega, Madison, WI). Other suitable vectors will be readily apparent to the skilled artisan. In an especially prefeoed embodiment, the recombinant vector comprises pCEPhABCl, which is described in Example 4 and shown in Figure 3. The recombinant vector pCEPhABCl comprises the plasmid pCEP4 (Invitrogen), an expression vector containing the cytomegalovirus promoter and enhancer. The vector pCEPhABCl further comprises an ABCl polynucleotide operatively linked to the heterologous cytomegalovirus promoter. The ABCl polynucleotide comprises SEQ ID NO: 1 , which contains the full-lenth ABCl cDNA, including non-coding 5' flanking (i.e., native ABCl promoter) and 3' flanking sequences. In addition, the present invention provides recombinant vectors comprising ABC 1 flanking sequence polynucleotides. In one embodiment, the recombinant vector comprises a polynucleotide comprising a ABCl 5' flanking sequence that preferably contains promoter activity. Thus, in a prefeoed embodiment, the recombinant vector comprises the polynucleotide comprising SEQ ID NO: 3. In another prefeoed embodiment, the recombinant vector comprises the polynucleotide comprising nucleotides 1-1532, 1080-1643, 1 181-1643, 1292- 1643, or 1394-1643 of SEQ ID NO: 3. In an especially prefeoed embodiment, the polynucleotide comprises nucleotides 1394-1532 of SEQ ID NO: 3. Also, in another embodiment, the recombinant vector comprises a polynucleotide that hybridizes, under stringent conditions, to the polynucleotide set forth in SEQ ID NO: 3 or the polynucleotide comprising nucleotides 1-1532, 1080-1643, 1 181-1643, 1292-1643, or 1394-1643 of SEQ ID NO: 3. In yet another embodiment, the polynucleotide comprises a nucleotide sequence that has at least 80%, more preferably at least 90%, and even more preferably at least 95% identity over its entire length to the polynucleotide comprising SEQ ID NO: 3 or the polynucleotide comprising nucleotides 1-1532, 1080-1643, 1 181-1643, 1292-1643, or 1394-1643 of SEQ ID NO: 3. The recombinant vector also comprises a polynucleotide that is complementary to any of the above- described 5' flanking sequences. In another embodiment, the recombinant vector comprises a polynucleotide comprising a 3' flanking sequence of ABCl . In a prefeoed embodiment, the recombinant vector comprises the polynucleotide comprising SEQ ID NO: 4. In another prefeoed embodiment, the recombinant vector comprises the polynucleotide comprising SEQ ID NO: 5. In an equally prefeoed embodiment, the recombinant vector comprises the polynucleotide comprising SEQ ID NO: 6. Also, in another embodiment, the recombinant vector comprises a polynucleotide that hybridizes, under stringent conditions, to the polynucleotide set forth in SEQ ID NO: 4, SEQ ID NO: 5, or SEQ ID NO: 6. In yet another embodiment, the polynucleotide comprises a nucleotide sequence that has at least 80%, more preferably at least 90%, and even more preferably at least 95% identity over its entire length to the polynucleotide comprising SEQ ID NO: 4, SEQ ID NO: 5, or SEQ ID NO: 6. The recombinant vector also comprises a polynucleotide that is complementary to any of the above-described 3' flanking sequences. An isolated ABCl flanking sequence polynucleotide, such as any of the above-described 5' or 3' flanking sequence polynucleotides, is inserted into a vector using well-known ligation and cloning techniques. Any of the previously described vectors can be used. Preferably, the vector is compatible with bacterial, insect, or mammalian host cells. Also preferably, the vector is an expression vector. The vector may be, for example, a phage, plasmid, viral, or retroviral vector. In addition to the ABCl flanking sequence, the vector may contain one or more of the following flanking nucleotide sequences: a promoter, one or more enhancer sequences, an origin of replication, a transcriptional termination sequence, a complete intron sequence containing a donor and acceptor splice site, a sequence encoding a leader sequence for polypeptide secretion, a ribosome binding site, a polyadenylation sequence, a polylinker region for inserting the nucleic acid encoding the polypeptide to be expressed, and a selectable marker element. Any of the previously described flanking nucleotide sequences are suitable. The flanking sequences may be homologous, heterologous, hybrid, or synthetic. Also, the flanking sequences may be native sequences which normally function to regulate ABCl polypeptide expression. The source of a flanking sequence may be any prokaryotic or eukaryotic organism, any vertebrate or invertebrate organism, or any plant, provided that the flanking sequence is functional in, and can be activated by, the host cell machinery. Prefeoed vectors are those which are compatible with bacterial, insect, and mammalian host cells. Suitable vectors have been previously described and include pQE70, pQE60, pQE9, pBluescript vectors, Phagescript vectors, pNH 16 A, pNH 18 A, pNH46 A, ptrc99a, pKK223 -3 , pDR540, pRIT5, and pCEP4 for use in bacteria and pWLNEO, pSV2CAT, pOG44, pXTI, pSG, pSVK3, pBPV, pMSG, pSVL, and pGL3 for use in eukaryotic cells. In one particularly prefeoed embodiment, the recombinant vector comprises a polynucleotide comprising the 5' flanking region of the ABCl gene and further comprises at least one polynucleotide encoding a heterologous polypeptide. The heterologous polynucleotide is operatively linked to the ABCl 5' flanking sequence. The ABCl 5' flanking sequence preferably contains the ABCl promoter. Thus, preferably, the 5' flanking sequence comprises the sequence set forth in SEQ ID NO: 3. Equally preferably, the 5' flanking sequence comprises nucleotides 1-1532, 1080-1643, 1 181-1643, 1292-1643, or 1394-1643 of SEQ ID NO: 3. The heterologous polynucleotide preferably encodes a polypeptide that is a complete protein or a biologically active fragment of a protein. The vector may also contain more than one heterologous polynucleotide. More preferably, the heterologous polynucleotide encodes a reporter protein. In such case, the recombinant vector preferably does not contain any additional promoter sequences. Examples of suitable reporter proteins include luciferase, β-galactosidase, chloramphenicol acetyl transferase transferase, and green fluorescent protein. Preferably, the reporter polypeptide is luciferase. Thus, in one especially prefeoed embodiment, the recombinant vector comprises the 5' flanking sequence set forth in SEQ ID NO: 3 and a luciferase reporter polynucleotide. In other equally prefeoed embodiments, the recombinant vector comprises a polynucleotide comprising nucleotides 1-1532, 1080-1643, 1 181-1643, 1292- 1643, or 1394-1643 of SEQ ID NO: 3 and a luciferase reporter polynucleotide. Expression vectors comprising the ABCl 5' flanking sequence can be constructed from a starting vector, such as a commercially available vector, which contains a reporter polynucleotide. Examples of suitable expression vectors include pGL3-Basic, which contains a luciferase reporter gene (Promega, Madison, WI) and pβGal-Basic (Clontech, Palo Alto, CA). A prefeoed vector is the pGL3-Basic luciferase reporter vector, which is promoterless. A 5' flanking sequence containing the ABCl promoter, for example SEQ ID NO: 3, can be ligated into one of the above expression vectors using well-known methods, including the methods described herein (see Example 15). Thus, in an especially prefeoed embodiment, the recombinant vector is pAPRl, a reporter gene construct comprising SEQ ID NO: 3 and a luciferase reporter gene in a pGL3 vector (see Figure 11). The present invention also relates to host cells comprising any one of the above-described recombinant vectors. After the vector has been constructed, the completed vector can be inserted into a suitable host cell for amplification and/or polypeptide expression. Host cells may be prokaryotic host cells (such as E. coli) or eukaryotic host cells (such as a yeast cell, an insect cell, or a vertebrate cell). The host cell, when cultured under appropriate conditions, synthesizes an ABCl polypeptide or, alternatively, a reporter polypeptide, which can be subsequently measured. The host cell can be a mammalian host cell, such as a primate cell line or a rodent cell line, including transformed cell lines. Normal diploid cells, cell strains derived from in vitro culture of primary tissue, as well as primary explants, are also suitable. Candidate host cells may be genotypically deficient in the selection gene, or may contain a dominantly acting selection gene. A number of suitable host cells are known in the art and many are available from the American Type Culture Collection (ATCC), Manassas, VA. Suitable mammalian host cells include, but are not limited to, Chinese hamster ovary cells (CHO), CO DHFR- cells (Urlaub et al, 1980, Proc. Natl. Acad. Sci., 97:4216-20), human embryonic kidney (HEK) 293 or 293T cells, or 3T3 cells, monkey COS-1 and COS-7 cell lines, and the CV-1 cell line. Other suitable mammalian cell lines include but are not limited to, mouse neuroblastoma N2A cells, HeLa cells, mouse 1-929 cells, 3T3 lines derived from Swiss, Balb-c or NIH mice, BHK or HAK hamster cell lines, Thp-1, HepG2, and mouse RAW cell lines. Each of these cell lines is known by and available to those skilled in the art of protein expression. A prefeoed host cell is the mouse monocytic cell line RAW 264.7, which use is described in Example 8. Suitable bacterial host cells include various strains of E. coli (e.g., HB101, DH50, DH10, and MCI 061), which are well-known as host cells in the field of biotechnology. Various strains of B. subtilis, Pseudomonas spp., other Bacillus spp., Streptomvces spp., and the like may also be employed in this method. Also, many strains of yeast cells are also available as host cells for the expression of ABCl polypeptides. Prefeoed yeast cells include, for example, Saccharomyces cerivisae and Pichia pastoris . Additionally, insect cell systems may be suitable host cells. Insect cell systems are described, for example, in Kitts et al, 1993, Biotechniques, 14:810-17; Lucklow, 1993, Curr. Opin. Biotechnol 4:564-72; and Lucklow et al, 1993, J. Virol, 67:4566- 79. Prefeoed insect cells are Sf-9 and Hi5 (Invitrogen). The present invention also provides a method for expressing an ABC 1 protein in a mammalian host cell comprising the steps of: (a) transfecting the mammalian host cell with a recombinant expression vector comprising a polynucleotide encoding ABCl in an amount sufficient to produce a detectable level of ABCl protein; and (b) purifying the produced ABCl protein. In one prefeoed embodiment, the recombinant expression vector comprises a polynucleotide encoding the polypeptide comprising SEQ ID NO: 2. In another prefeoed embodiment, the polynucleotide comprising SEQ ID NO: 1. In yet another prefeoed embodiment, the polynucleotide comprising nucleotides 291-7074 of SEQ ID NO: 1. In still another prefeoed embodiment, the polynucleotide encoding a polypeptide that is at least 98% identical to the polypeptide comprising SEQ ID NO: 2. Introduction of the recombinant ABCl vector into a mammalian host cell can be effected by methods well-known in the art and described in standard laboratory manuals, such as Sambrook, supra. Preferably, the recombinant vector is introduced into a host cell in a precipitate or in a complex with a charged lipid. Suitable methods for introduction of the ABCl vector include calcium phosphate transfection, DEAE-dextran mediated transfection, cationic lipid-mediated transfection, electroporation, transduction, infection, or other methods known in the art. These methods are described, for example, in Davis et al., Basic Methods in Molecular Biology (1986). If the recombinant ABCl vector is a viral vector, it may be packaged in vitro using an appropriate packaging cell line and then transduced into host cells. The ABCl polypeptide can be recovered and purified from recombinant cell cultures using well-known methods, including ammonium sulfate or ethanol precipitation, acid, extraction, anion or cation exchange chromatography, phosphocellulose chromatography, hydrophobic interaction chromatography, affinity chromatography, hydroxylapatite chromatoraphy, and lectin chromatography (see, e.g., Smith and Johnson, Gene 67:31-40 (1988)). Preferably, affinity chromotography using anti-ABC 1 antibodies is employed for purification. In addition, the present invention provides a method for expressing ABCl protein in a mammalian subject comprising the step of administering to a mammalian subject a recombinant expression vector comprising a polynucleotide encoding ABCl in an amount sufficient to express ABCl protein in said mammalian subject. Preferably, the recombinant expression vector comprises a polynucleotide encoding the polypeptide comprising SEQ ID NO: 2, the polynucleotide comprising SEQ ID NO: 1, the polynucleotide comprising nucleotides 291-7074 of SEQ ID NO: 1, or the polynucleotide encoding a polypeptide that is at least 98% identical to the polypeptide comprising SEQ ID NO: 2. Introduction of the recombinant ABCl vector into a mammalian subject can be effected by methods well-known in the art and are described in detail herein below. Expression of ABCl can be measured by obtaining a blood sample from the subject to whom the recombinant ABCl vector was administered, separating the monocyte population, and measuring the ABCl gene expression in macrophage cells. The ABCl gene expression can be measured using methods well-known in the art, such as RT-PCR, and methods described herein (see Examples 9 and 10). The level of ABCl protein can be measured by obtaining a blood sample from the subject, separating the monocyte population and measuring the ABCl protein in macrophage cells using well-known methods, such as immunoprecipitation, described herein at Example 1 1. Methods and Compounds for Increasing Cholesterol Efflux In another aspect of the present invention, a method suitable for increasing cholesterol efflux from cells in a mammalian subject is provided. Such method comprises administering to the mammalian subject a recombinant expression vector comprising an ABC l polynucleotide in an amount sufficient to increase cholesterol efflux from said cells. The recombinant vector can be any of the above-described vectors containing any of the previously described wild-type or variant ABC l polynucleotides, as long as the encoded ABC l polypeptide has biological activity (i.e., cholesterol transport activity). Preferably, the recombinant vector comprises the polynucleotide encoding the polypeptide comprising SEQ ID NO: 2, the polynucleotide comprising SEQ ID NO: 1 , the polynucleotide comprising nucleotides 291-7074 of SEQ ID NO: 1 , or the polynucleotide encoding a polypeptide comprising an amino acid sequence which is at least 98% identical to the amino acid sequence of SEQ ID NO: 2. Also, preferably, the recombinant vector comprises a variant polynucleotide that is at least 80%, 90%, or 95% identical to the polynucleotide comprising SEQ ID NO: 1 , as long as the encoded ABC l polypeptide has cholesterol transport activity. The administration of a recombinant ABC l expression vector to a mammalian subject can be used to express the ABCl gene in said subject for the treatment of cardiovascular disease. Specifically, this method would achieve its therapeutic effect by the introduction of the ABCl gene into macrophage cells and other cholesterol-accumulating cells found in the arterial lesions of mammals with cardiovascular disease. Expression of the ABCl polynucleotide in target cells would effect greater production of the ABCl protein. The subsequently produced ABC l protein would ameliorate the disease by increasing the efflux of cholesterol from macrophage and other cholesterol-laden cells found in arterial lesions onto nascent HDL particles. The cellular efflux would lead to the overall removal of cholesterol from peripheral sites, such as the cholesterol- rich core of arterial plaques. A concuoent reduction in the size of these pathological lesions reduces the risk of arterial blockage that leads to heart attacks and angina. This method could also be used prophylactically to prevent the accumulation of cholesterol in arterial walls. A sufficient amount of ABC l expression vector is the amount of ABCl vector that increases cholesterol efflux from the cells of a mammalian subject. Such amount can be determined by measuring the cholesterol efflux in the cells of a subject before (control level) and after administration of the recombinant ABCl expression vector at various dosages and determining the dose that effects an increase in cholesterol efflux compared to control level. The cholesterol efflux can be measured by obtaining a blood sample from the subject, separating the monocyte population, and measuring the amount of cholesterol efflux in a subject's macrophage cells. Any of the assays described herein can be used to measure cholesterol efflux. In addition, cholesterol efflux can be measured by determining the level of plasma HDL- cholesterol in a subject before (control) and after administration of a recombinant ABCl expression vector. An observed increase in HDL-cholesterol in the serum of a subject following administration of a recombinant ABC 1 expression vector indicates an increase in cholesterol efflux. HDL-cholesterol levels in serum can be determined using methods well-known in the art. In addition, cholesterol efflux can be measured by measuring the size of atherosclerotic lesions found in the arterial wall of a subject before (control level) and after administration of the recombinant ABCl expression vector. A reduction in the size of the arterial lesion indicates an increase in cholesterol efflux. Assays for measuring arterial lesions are well known in the art. For example, increased cholesterol efflux from arterial lesions can be measured using the mouse model of atherosclerosis described in Lawn et al., Nature, 360: 670-672 (1992)), as well as any of the other known models. Using the LDL receptor knockout mouse described in Lawn et al., cholesterol efflux can be measured before and after administration of the ABCl vector. Fatty streak lesion size in groups of animals fed an atherogenic diet can be measured by oil-red O staining of aortic sections as described in Lawn et al. A reduction in size of fatty streak lesions in the group receiving the ABC 1 expression vector compared to a group receiving a control vector indicates an increase in cholesterol efflux from the lesions. In humans, the size of atherosclerotic lesions found in arterial walls can be measured using, for example, angiography and non-invasive ultrasound methods. Alternatively, cholesterol efflux can be measured by obtaining a blood sample and measuring the level of ABCl mRNA or ABCl protein in the macrophage cells of a subject before and after administration of a recombinant ABCl expression vector. Routine assays can be performed to determine the cooelation between increasing ABC 1 mRNA concentrations and cholesterol efflux. Likewise, assays can be performed to cooelate the amount of ABCl protein with the amount of cholesterol efflux. Using such cooelation data, an observed increase in ABCl mRNA or ABCl protein in the cells of a subject following administration of a recombinant ABCl expression vector can be used to indicate an increase in cholesterol efflux. ABC 1 mRNA and ABC 1 protein levels can be measured using the assays described herein and other well-known techniques for mRNA and protein quantitation. Therapeutic dosages and formulations are discussed in further detail below. There are available to one skilled in the art multiple viral and non-viral methods suitable for introduction of a nucleic acid molecule into a target cell. For example, viral delivery vectors suitable for gene therapy include, but are not limited to. adenovirus, herpes simplex virus, pox virus (i.e., vaccinia), hepatitis virus, parvovirus, papovavirus, alphavirus, coronavirus, rhabdovirus, papilloma virus, adeno-associated virus (AAV), polio virus, and RNA viruses, such as a retroviruses and Sindbis virus. The ABCl polynucleotide can also be delivered using a non- viral delivery system, such as naked DNA delivery (direct injection), receptor-mediated transfer (DNA-ligand complexes), electroporation, adenovirus-ligand-DNA complexes, calcium phosphate (CaP0 4 ) precipitation, microparticle bombardment (gene gun techniques), liposome- mediated transfer, and lipofection. Genetic modification of a cell may be accomplished using one or more techniques well known in the gene therapy field (Mulligan, R., 1993, Science, 260 (51 10): 926-32). Gene therapy materials and methods can include inducible promoters, tissue-specific enhancer- promoters, DNA sequences designed for site-specific integration, DNA sequences capable of providing a selective advantage over the parent cell, labels to identify transformed cells, negative selection systems and expression control systems (safety measures), cell-specific binding agents (for cell targeting), cell-specific internalization factors, and transcription factors to enhance expression by a vector as well as methods of vector manufacture. Examples of methods and materials for the practice of gene therapy techniques are described in U.S. Patent Nos. 4,970,154 (involving electroporation techniques), 5,679,559 (describing a lipoprotein-containing system for gene delivery), 5,676,954 (involving liposome carriers), 5,593,875 (describing methods for calcium phosphate transfection), and 4,945,050 (describing a process wherein biologically active particles are propelled at cells at a speed whereby the particles penetrate the surface of the cells and become incorporated into the interior of the cells), and PCT Pub. No. WO 96/40958 (involving nuclear ligands). Adenoviral vectors have proven especially useful for gene transfer into eukaryotic cells (Rosenfeld, M., et al, Science, 252: 431-4 (1991); U.S. Patent No. 5,631,236). The first trial of Ad-mediated gene therapy in human was the transfer of the cystic fibrosis transmembrane conductance regulator (CFTR) gene to lung (Crystal, R., et al, 1994, Nat. Genet., 8 (1): 42-51). Experimental routes for administrating recombinant Ad to different tissues in vivo have included intratracheal instillation (Rosenfeld, M., et al, 1992, Cell, 68 (1): 143-55) injection into muscle (Quantin, B., et al, 1992, Proc. Natl Acad. Sci. U.S.A., 89 (7): 2581-4), peripheral intravenous injection (Herz, J., and Gerard, R., 1993, Proc. Natl. Acad. Sci. U.S.A., 90 (7): 2812-6) and stereotactic inoculation to brain (Le Gal La Salle, G., et al, 1993, Science, 259 (5097): 988-90). The adenoviral vector, then, is widely available to one skilled in the art and is suitable for use in the present invention. Adeno-associated virus (AAV) has recently been introduced as a gene transfer system with potential applications in gene therapy. Wild-type AAV demonstrates high-level infectivity, broad host range and specificity in integrating into the host cell genome (Hermonat, P., and Muzyczka, N., 1984, Proc. Natl. Acad. Sci. U.S.A., 81 (20): 6466-70). Herpes simplex virus type-1 (HSV-1) is a prefeoed vector system (Geller, A., et al, 1991 , Trends Neurosc , 14 (10): 428-32; Glorioso, J., et al, 1995, Mol. Biotechnol, 4 (1): 87-99; Glorioso, J., et al, 1995, Annu. Rev. Microbiol, 49: 675-710). Vaccinia virus, of the poxvirus family, has also been developed as an expression vector (Smith, G., and Moss, B., 1983, Gene, 25 (1): 21-8; Moss, B., 1992, Biotechnology, 20: 345-62; Moss, B., 1992, Cwrr. Top. Microbiol Immunol, 158: 25-38). Each of the above-described vectors is widely available to one skilled in the art and would be suitable for use in the present invention. Preferably, the viral delivery system utilizes a retroviral vector. Retroviral vectors are capable of infecting a large percentage of the target cells and integrating into the cell genome (Miller, A., and Rosman, G., Biotechniques, 7(9): 980-2, 984-6, 989-90 (1989); U.S. Patent No. 5,672,510). Retroviruses were developed as gene transfer vectors relatively earlier than other viruses, and were first used successfully for gene marking and transducing the cDNA of adenosine deaminase (ADA) into human lymphocytes. Preferably, the retroviral vector is a derivative of a murine or avian retrovirus, or is a lentiviral vector. An especially prefeoed retroviral vector is a lentiviral vector. Examples of retroviral vectors in which a single foreign gene can be inserted include, but are not limited to: Moloney murine leukemia virus (MoMuLV), Harvey murine sarcoma virus (HaMuSV), murine mammary tumor virus (MuMTV), SIV, BIV, HIV and Rous Sarcoma Virus (RSV). A number of additional retroviral vectors can incorporate multiple genes. All of these vectors can transfer or incorporate a gene for a selectable marker so that transduced cells can be identified and generated. Since recombinant retroviruses are defective, they require assistance in order to produce infectious vector particles. This assistance can be provided, for example, by using helper cell lines that contain plasmids encoding all of the structural genes of the retrovirus under the control of regulatory sequences within the LTR. The helper plasmids are missing a nucleotide sequence that enables the packaging mechanism to recognize an RNA transcript for encapsitation. Thus, helper cell lines produce empty virions, since no genome is packaged. Suitable helper cell lines include, but are not limited to Ψ2, PA317 and PA 12. If a retroviral vector is introduced into such cells in which the packaging signal is intact, but the structural genes are replaced by other genes of interest, the vector can be packaged and vector virion produced. The vector virions produced by this method can then be used to infect a tissue cell line, such as NIH 3T3 cells, to produce large quantities of chimeric retroviral virions. The vectors of the present invention may be constructed using standard recombinant techniques widely available to one skilled in the art. Such techniques may be found in common molecular biology references such as Molecular Cloning: A Laboratory Manual (Sambrook, et al., 1989, Cold Spring Harbor Laboratory Press), Gene Expression Technology (Methods in Enzymology, Vol. 185, edited by D. Goeddel, 1991 , Academic Press, San Diego, CA), and ECR Protocols: A Guide to Methods and Applications (Innis, et al. 1990, Academic Press, San Diego, CA). In order to obtain transcription of the ABCl polynucleotide within a target cell, a transcriptional regulatory region capable of driving gene expression in the target cell must be utilized. The transcriptional regulatory region preferably comprises a promoter and/or enhancer, which is operatively linked to the ABCl polynulceotide. The promoter can be homologous or heterologous to the ABCl gene, provided that it is active in the cell or tissue-type into which the construct will be inserted. The transcriptional regulatory region chosen should drive high level gene expression in the target cell. Preferably, a macrophage-specific promoter, such as a scavenger receptor type A, matrix metalloproteinase promoter (MMP-12), or macrophage-tropic lentivirus promoter (Fabunmi et al., Atherosclerosis, 148: 375-386 (1999)), is used. A particularly prefeoed promoter is the 5' region of the scavenger receptor type A gene, which contains a strong macrophage promoter that can be used to drive the transcription of the ABCl gene. In addition, a means to increase endogenous ABCl polypeptide expression in a cell is to insert one or more enhancer elements into the promoter region, where the enhancer elements can serve to increase transcriptional activity of the ABCl gene. Similarly, the enhancer element(s) used is selected based on the tissue in which one desires to activate the gene. Thus, for example, enhancer elements known to confer promoter activation in cells found in arterial tissue, especially macrophage cells, will be selected. Other transcriptional regulatory regions suitable for use in the present invention include but are not limited to the human cytomegalovirus (CMV) immediate-early enhancer/promoter, the SV40 early enhancer/promoter, the JC polyomavirus promoter, the albumin promoter, PGK and the α-actin promoter coupled to the CMV enhancer (Doll, R., et al, 1996, Gene Ther., 3 (5): 437-47). Other components of the vector construct may optionally include DNA molecules designed for site-specific integration (e.g., endogenous sequences useful for homologous recombination), tissue-specific promoters, DNA molecules capable of providing a selective advantage over the parent cell, DNA molecules useful as labels to identify transformed cells, negative selection systems, cell specific binding agents (e.g., for cell targeting), cell-specific internalization factors, transcription factors enhancing expression from a vector, and factors enabling vector production. In one embodiment, the vector can include targeting DNA for site-specific integration. The targeting DNA is a nucleotide sequence that is complementary (homologous) to a region of the gene of interest, for example, the ABCl gene. Through homologous recombination, the DNA sequence to be inserted into the genome can be directed to the ABC 1 gene by attaching it to the targeting DNA. DNA sequences for insertion include, for example, regions of DNA that may interact with or control the expression of an ABCl polypeptide, e.g., flanking sequences. Thus, the expression of the desired ABCl polypeptide is achieved not by transfection of DNA that encodes the ABCl gene itself, but rather by the use of targeting DNA coupled with DNA regulatory segments that provide the endogenous gene sequence with recognizable signals for transcription of an ABCl polypeptide (Sauer, Curr. Opin. Biotechnol, 5:521-27 (1994); Sauer, Methods Enzymol, 225:890-900 (1993)). In yet other embodiments, regulatory elements can be included for the controlled expression of the ABCl gene in the target cell. Such elements are turned on in response to an appropriate effector. In this way, a therapeutic ABCl polypeptide can be expressed when desired. One conventional control means involves the use of small molecule dimerizers or rapalogs to dimerize chimeric proteins which contain a small molecule-binding domain and a domain capable of initiating a biological process, such as a DNA-binding protein or transcriptional activation protein (see PCT Pub. Nos. WO 96/41865, WO 97/31898, and WO 97/31899). The dimerization of the proteins can be used to initiate transcription of the transgene. Another suitable control means or gene switches includes the use of mifepristone (RU486), which is a progesterone antagonist. The binding of a modified progesterone receptor ligand- binding domain to the progesterone antagonist activates transcription by forming a dimer of two transcription factors that then pass into the nucleus to bind DNA. The ligand-binding domain is modified to eliminate the ability of the receptor to bind to the natural ligand. The modified steroid hormone receptor system is further described in U.S. Patent No. 5,364,791 and PCT Pub. Nos. WO 96/4091 1 and WO 97/10337. Yet another control means uses a positive tetracycline- controllable transactivator. This system involves a mutated tet repressor protein DNA-binding domain (mutated tet R-4 amino acid changes which resulted in a reverse tetracycline-regulated transactivator protein, i.e., it binds to a tet operator in the presence of tetracycline) linked to a polypeptide which activates transcription. Such systems are described in U.S. Patent Nos. 5,464,758, 5,650,298. and 5,654, 168. Additional expression control systems and nucleic acid constructs are described in U.S. Patent Nos. 5,741,679 and 5,834,186. Viral delivery vectors containing an ABCl polynucleotide can be made target specific by altering the viral coat such that it contains a ligand that is specific for another molecule found on the target cell. This will allow the vector to bind specifically to the desired cell-type. The ligand can be any compound of interest that will bind specifically to a molecule found on the target cell, such as a cell-surface receptor. Preferably, the receptor is found exclusively on target cells and not on other cells. For example, ligands for scavenger receptor A can be used to direct viral delivery vectors to macrophage cells. Alternatively, the viral coat can be altered such that it contains an antibody or antibody fragment, such as Fab, or F(ab') 2 , that recognizes and binds to an antigenic epitope on the target cells. The viral coat can be altered by inserting an additional polynucleotide that encodes the ligand into the viral genome. Those of skill in the art will know of other specific polynucleotide sequences which can be inserted into the viral genome to allow target specific delivery of the vector containing the ABCl polynucleotide. In addition, a naked ABCl polynucleotide can be administered. ABCl polynucleotides and recombinant ABCl expression vectors, such as those described above, can be administered as a pharmaceutical composition. Such a composition comprises an effective amount of an ABCl polynucleotide or recombinant ABCl expression vector, as previously defined herein, and a pharmaceutically acceptable formulation agent selected for suitability with the mode of administration. Suitable formulation materials preferably are non-toxic to recipients at the concentrations employed and are described herein below. The phaonaceutical composition comprising an ABCl polynucleotide or an ABCl recombinant expression vector may contain formulation materials for modifying, maintaining, or preserving, for example, the pH, osmolarity, viscosity, clarity, color, isotonicity, odor, sterility, stability, rate of dissolution or release, adsorption, or penetration of the composition. Suitable formulation materials include, but are not limited to, amino acids (such as glycine, glutamine, asparagine, arginine, or lysine), antimicrobials, antioxidants (such as ascorbic acid, sodium sulfite, or sodium hydrogen-sulfite), buffers (such as borate, bicarbonate, Tris-HCl, citrates, phosphates, or other organic acids), bulking agents (such as mannitol or glycine), chelating agents (such as ethylenediamine tetraacetic acid (EDTA)), complexing agents (such as caffeine, polyvinylpyoolidone, beta-cyclodextrin, or hydroxypropyl-beta-cyclodextrin), fillers, monosaccharides, disaccharides. and other carbohydrates (such as glucose, mannose, or dextrins), proteins (such as serum albumin, gelatin, or immunoglobulins), coloring, flavoring and diluting agents, emulsifying agents, hydrophilic polymers (such as polyvinylpyoolidone), low molecular weight polypeptides, salt-forming counterions (such as sodium), preservatives (such as benzalkonium chloride, benzoic acid, salicylic acid, thimerosal, phenethyl alcohol, methylparaben, propylparaben, chlorhexidine, sorbic acid, or hydrogen peroxide), solvents (such as glycerin, propylene glycol, or polyethylene glycol), sugar alcohols (such as mannitol or sorbitol), suspending agents, surfactants or wetting agents (such as pluronics; PEG; sorbitan esters; polysorbates such as polysorbate 20 or polysorbate 80; triton; tromethamine; lecithin; cholesterol or tyloxapal), stability enhancing agents (such as sucrose or sorbitol), tonicity enhancing agents (such as alkali metal halides - preferably sodium or potassium chloride - or mannitol sorbitol), delivery vehicles, diluents, excipients and/or pharmaceutical adjuvants. See Remington 's Pharmaceutical Sciences (18 th Ed., A.R. Gennaro, ed., Mack Publishing Company 1990). The pharmaceutically active compounds (i.e. ABCl polynucleotide or ABCl vector) can be processed in accordance with conventional methods of pharmacy to produce medicinal agents for administration to patients, including humans and other mammals. Thus, the pharmaceutical composition comprising an ABCl polynucleotide or an ABCl recombinant expression vector may be made up in a solid form (including granules, powders or suppositories) or in a liquid form (e.g., solutions, suspensions, or emulsions). Solid dosage forms for oral administration may include capsules, tablets, pills, powders, and granules. In such solid dosage forms, the active compound may be admixed with at least one inert diluent such as sucrose, lactose, or starch. Such dosage forms may also comprise, as in normal practice, additional substances other than inert diluents, e.g. , lubricating agents such as magnesium stearate. In the case of capsules, tablets, and pills, the dosage forms may also comprise buffering agents. Tablets and pills can additionally be prepared with enteric coatings. Liquid dosage forms for oral or parenteral administration may include pharmaceutically acceptable emulsions, solutions, suspensions, syrups, and elixirs containing inert diluents commonly used in the art, such as water. Such compositions may also comprise adjuvants, such as wetting sweetening, flavoring, and perfuming agents. For example, a suitable carrier for injection may be water, physiological saline solution, or artificial cerebrospinal fluid, possibly supplemented with other materials common in compositions for parenteral administration. Neutral buffered saline or saline mixed with serum albumin are further exemplary carriers. Other exemplary pharmaceutical compositions comprise Tris buffer of about pH 7.0-8.5, or acetate buffer of about pH 4.0-5.5, which may further include sorbitol or a suitable substitute. The dosage regimen for treating a cardiovascular disease with a composition comprising an ABCl polynucleotide or ABCl expression vector is based on a variety of factors, including the type of cardiovascular disease, the age, weight, sex, medical condition of the patient, the severity of the condition, the route of administration, and the particular compound employed. Thus, the dosage regimen may vary widely, but can be determined routinely using standard methods. For example, the amount of ABCl polynucleotide or ABCl expression vector to be administered is an amount sufficient to increase cholesterol efflux from the cells of a mammalian subject. Such amount can be determined, for example, by measuring the plasma HDL- cholesterol level of a subject before and after administration of the ABCl polynucleotide or ABCl expression vector. A sufficient amount of ABCl polynucleotide or ABCl expression vector is an amount that increases the plasma HDL-cholesterol level of a subject. Accordingly, the clinician can titer the dosage and modify the route of administration to obtain the optimal therapeutic effect. A typical dosage may range from about 0.1 ?g/kg to about 100 mg kg or more, depending on the factors mentioned above. The frequency of dosing will depend upon the pharmacokinetic parameters of the ABCl polynucleotide or vector in the formulation being used. Typically, a clinician will administer the composition until a dosage is reached that achieves the desired effect. The composition may therefore be administered as a single dose, as two or more doses (which may or may not contain the same amount of the desired molecule) over time, or as a continuous infusion via implantation device or catheter. Further refinement of the appropriate dosage is routinely made by those of ordinary skill in the art and is within the ambit of tasks routinely performed by them. Appropriate dosages may be ascertained through use of appropriate dose-response data. The cells of a mammalian subject may be transfected in vivo, ex vivo, or in vitro. Administration of an ABCl polynucleotide or a recombinant vector containing an ABCl polynucleotide to a target cell in vivo may be accomplished using any of a variety of techniques well known to those skilled in the art. For example, U.S. Patent No. 5,672,344 describes an in vivo viral-mediated gene transfer system involving a recombinant neurotrophic HS V- 1 vector. The above-described compositions of ABCl polynucleotides and recombinant ABCl vectors can be transfected in vivo by oral, buccal, parenteral, rectal, or topical administration as well as by inhalation spray. The term "parenteral" as used herein includes, subcutaneous, intravenous, intramuscular, intrasternal, infusion techniques or intraperitoneally. For oral administration, the pharmaceutical composition containing the ABCl polynucleotide or recombinant ABCl vector may be in the form of, for example, a capsule, a tablet, a suspension, or liquid. The pharmaceutical composition is preferably made in the form of a dosage unit containing a given amount of DNA or viral vector particles. For example, these may contain an amount from about 10 3 -10 15 viral particles, preferably from about 10 6 -10 12 viral particles. A suitable daily dose for a human or other mammal may vary widely depending on the condition of the patient and other factors, but, once again, can be determined using routine methods. The pharmaceutical composition containing the ABCl DNA or recombinant ABCl vector can also be administered rectally. Suitable suppositories for rectal administration of the vector can be prepared by mixing the vector with a suitable non-irritating excipient such as cocoa butter and polyethylene glycols that are solid at ordinary temperatures but liquid at the rectal temperature and will therefore melt in the rectum and release the vector. A pharmaceutical composition also can be formulated for inhalation. For example, an ABCl polynucleotide or vector may be formulated as a dry powder for inhalation. Also, ABCl polynucleotide or vector inhalation solutions can be formulated with a propellant for aerosol delivery. In yet another embodiment, solutions may be nebulized. The pharmaceutical composition containing the ABCl DNA or recombinant ABCl vector can also be injected. Injectable preparations, such as sterile injectable aqueous or oleaginous suspensions, may be formulated according to the known methods using suitable dispersing or wetting agents and suspending agents. The sterile injectable preparation may also be a sterile injectable solution or suspension in a non- toxic parenterally acceptable diluent or solvent, for example as a solution in 1,3-butanediol. A particularly suitable carrier for parenteral injection is sterile distilled water in which an ABCl polynucleotide or vector is formulated as a sterile, isotonic solution, properly preserved. Among the other acceptable carriers and solvents that may be employed are Ringer's solution, and isotonic sodium chloride solution. In addition, sterile, fixed oils are conventionally employed as a solvent or suspending medium. For this purpose any bland fixed oil may be employed, including synthetic mono- or diglycerides. In addition, fatty acids such as oleic acid find use in the preparation of injectables. Yet another preparation can involve the formulation of the desired ABCl molecule with an agent, such as injectable microsperes, bio-erodible particles, polymeric compounds, beads or liposomes, that provides for the controlled or sustained release of the ABCl product (see, e.g. PCT/US93/00829; Eppstein et al., Proc. Natl. Acad. Sci., 82: 3688-3692 (1985)). Also, the pharmaceutical composition containing the ABCl DNA or recombinant ABCl vector can administered topically. For topical administration, the vector may comprise from 0.001% to 10% w/w, e.g., from 1% to 2% by weight of the formulation, although it may comprise as much as 10% w/w, but preferably not more than 5% w/w, and more preferably from 0.1% to 1% of the formulation. Foonulations suitable for topical administration include liquid or semi-liquid preparations suitable for penetration through the skin (e.g. , liniments, lotions, ointments, creams, or pastes) and drops suitable for administration to the eye, ear, or nose. A suitable topical dose of active ingredient of a vector of the present invention is administered one to four, preferably two or three times daily. While the nucleic acids and /or vectors of the invention can be administered as the sole active pharmaceutical agent, they can also be used in combination with one or more vectors of the invention or other agents. When administered as a combination, the therapeutic agents can be formulated as separate compositions that are given at the same time or different times, or the therapeutic agents can be given as a single composition. In another embodiment of the present invention, a target cell is transfected in vivo by implantation of a "producer cell line" in proximity to the target cell population (Culver, K., et al, 1994, Hum. Gene Ther., 5 (3): 343-79; Culver, K., et al, Cold Spring Harb. Symp. Quant. Biol, 59: 685-90; Oldfield, E., 1993, Hum. Gene Ther., 4 (1): 39-69). The producer cell line is engineered to produce a viral vector containing the ABCl polynucleotide and to release its viral particles in the vicinity of the target cells, i.e. preferably macrophage cells found in atherosclerotic lesions. A portion of the released viral particles contact the target macrophage cells and infect those cells, thus delivering an ABC 1 polynucleotide to the target macrophage cell. Following infection of the target cell, expression of the ABCl polynucleotide occurs, providing the macrophage cell with functional ABCl protein. In another embodiment, the invention provides a method of treating a cardiovascular disease by the ex vivo introduction of an ABCl polynucleotide or recombinant ABCl expression vector. In such instances, cells, tissues, or organs that have been removed from the patient are exposed to ABCl compositions after which the cells, tissues, or organs are subsequently implanted back into the patient. For example, one method includes the removal of a blood sample from a subject with cardiovascular disease, enriching the sample for monocytes, and contacting the isolated monocytes with a recombinant expression vector containing the ABCl polynucleotide and, optionally, a target specific gene. Optionally, the monocyte cells can be treated with a growth factor, such as GM-CSF, to stimulate cell growth, before reintroducing the cells into the subject. When reintroduced, the cells will specifically target the cell population from which they were originally isolated. In this way, the transport activity of the ABCl polypeptide may be used to promote cholesterol efflux in a subject. Another method of ex vivo administration involves introducing the ABCl polynucleotide or recombinant ABC 1 vector into the mammalian subject by means of skin transplants of cells containing the virus. Preferably, a retrovius used for this method of administration. Long term expression of foreign genes in implants, using cells of fibroblast origin, may be achieved if a strong housekeeping gene promoter is used to drive transcription. For example, the dihydrofolate reductase (DHFR) gene promoter may be used. Cells such as fibroblasts, can be infected with virions containing a retroviral construct containing the ABCl polynucleotide together with a gene which allows for specific targeting, such as scavenger receptor A, and a strong promoter. The infected cells can be embedded in a collagen matrix that can be grafted into the connective tissue of the dermis in the recipient subject. As the retrovirus proliferates and escapes the matrix it will specifically infect the target cell population. In this way the transplantation results in increased amounts of cholesterol efflux activity in cells manifesting the transport disorder. In another embodiment, the recombinant expression vector comprising the ABC 1 polynucleotide can be administered using in vitro techniques, such as described in U.S. Patent No. 5,399,346. For example, an ABCl polypeptide can be delivered by implanting certain cells that have been genetically engineered, using methods such as those described herein, to express the ABCl polypeptide. In order to minimize a potential immunological reaction in patients being administered an ABCl polypeptide, as may occur with the administration of a polypeptide of a foreign species, it is prefeoed that the natural cells producing ABCl polypeptide be of human origin and produce human ABCl polypeptide. Thus, it is prefeoed that the recombinant cells producing ABCl polypeptide be transformed with an expression vector containing a gene encoding a human ABCl polypeptide. The cells can be autologous or heterologous. Optionally, the cells can be immortalized. In order to decrease the chance of an immunological response, the cells may be encapsulated to avoid infiltration of suoounding tissues. The encapsulation materials are typically biocompatible, semi-permeable polymeric enclosures or membranes that allow the release of the protein product(s) but prevent the destruction of the cells by the patient's immune system or by other detrimental factors from the suoounding tissues. The transfected cells can be administered to a patient using the above-described methods. Techniques for the encapsulation of living cells are known in the art, and the preparation of the encapsulated cells and their implantation in patients may be routinely accomplished. For example, Baetge et al. (PCT Pub. No. WO 95/05452 and PCT/US94/09299) describe membrane capsules containing genetically engineered cells for the effective delivery of biologically active molecules. The capsules are biocompatible and are easily retrievable. The capsules encapsulate cells transfected with recombinant DNA molecules comprising DNA sequences coding for biologically active molecules operatively linked to promoters that are not subject to down regulation in vivo upon implantation into a mammalian host. The devices provide for the delivery of the molecules from living cells to specific sites within a recipient. In addition, see U.S. Patent Nos. 4,892,538; 5,01 1 ,472; and 5,106,627. A system for encapsulating living cells is described in PCT Pub. No. WO 91/10425 (Aebischer et al). See also, PCT Pub. No. WO 91/10470 (Aebischer et al); Winn et al, 1991, Exper. Neurol 1 13:322-29; Aebischer et al, 1991, Exper. Neurol. 1 1 1 :269-75; and Tresco et al, 1992, ASAIO 38: 17-23. Another delivery system for polynucleotides encoding ABCl is a "non- viral" delivery system. Techniques that have been used or proposed for gene therapy include DNA-ligand complexes, adenovirus-ligand-DNA complexes, direct injection of DNA, CaPO 4 precipitation, gene gun techniques, electroporation, lipofection, and colloidal dispersion (Mulligan, R., 1993, Science, 260 (51 10): 926-32). Any of these methods are widely available to one skilled in the art and would be suitable for use in the present invention. Other suitable methods are available to one skilled in the art, and it is to be understood that the present invention may be accomplished using any of the available methods of transfection. Several such methodologies have been utilized by those skilled in the art with varying success (Mulligan, R., 1993, Science, 260 (51 10): 926-32). Preferably, the non-viral delivery system is a colloidal dispersion system. Colloidal dispersion systems include macromolecule complexes, nanocapsules, microspheres, beads, and lipid-based systems including oil-in-water emulsions, micelles, mixed micelles, and liposomes. The prefeoed colloidal system of this invention is a liposome, which is an artificial membrane vesicle useful as delivery vehicles in vitro and in vivo. Liposomes are self-assembling, colloidal particles in which a lipid bilayer, composed of amphiphilic molecules such as phosphatidyl serine or phosphatidyl choline, encapsulates a portion of the suoounding media such that the lipid bilayer suoounds a hydrophilic interior. Unilammellar or multilammellar liposomes can be constructed such that the interior contains a desired chemical, drug, or, as in the instant invention, an isolated DNA molecule. For example, it has been shown that large unilamellar vesicles (LUV), which range in size from 0.2-4.0 μm can encapsulate a substantial percentage of an aqueous buffer containing large macromolecules, such as RNA, DNA and intact virions. Once encapsulated within the aqueous interior, these macromolecules can be delivered to mammalian cells in a biologically active form (Fraley, R. and Papahadjopoulos, D. 1981, Trends Biochem. Sci., 6: 77- 80). In order for a liposome to be an efficient gene transfer vehicle, the following characteristics should be present: (1) encapsulation of the genes of interest at high efficiency while not compromising their biological activity; (2) preferential and substantial binding to a target cell in comparison to non- target cells; (3) delivery of the aqueous contents of the vesicle to the target cell cytoplasm at high efficiency; and (4) accurate and effective expression of genetic information (Mannino, R., et al, 1988, Biotechniques, 6 (7): 682-90). The composition of the liposome is usually a combination of phospholipids, particularly high-phase-transition-temperature phospholipids, usually in combination with steroids, especially cholesterol. Other phospholipids or other lipids may also be used. The physical characteristics of liposomes depend on pH, ionic strength, and the presence of divalent cations. Examples of lipids useful in liposome production include phosphatidyl compounds, such as phosphatidylglycerol, phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, sphingolipids, cerebrosides, and gangliosides. Particularly useful are diacylphosphatidylglycerols, where the lipid moiety contains from 14-18 carbon atoms, particularly from 16-18 carbon atoms, and is saturated. Illustrative phospholipids include egg phosphatidylcholine, dipalmitoylphosphatidylcholine and distearoylphosphatidylcholine. The targeting of liposomes has been classified based on anatomical and mechanistic factors. Anatomical classification is based on the level of selectivity, for example, organ- specific, cell-specific, and organelle-specific. Mechanistic targeting can be distinguished based upon whether it is passive or active. Passive targeting utilizes the natural tendency of liposomes to distribute to cells of the reticulo-endothelial system (RES) in organs which contain sinusoidal capillaries. Active targeting, on the other hand, involves alteration of the liposome by coupling the liposome to a specific ligand, such as a polyclonal or monoclonal antibody, sugar, glycolipid, or protein, or by changing the composition or size of the liposome in order to achieve targeting to organs and cell types other than the naturally occurring sites of localization. Preferably, the ligand is a polyclonal or monoclonal antibody which can be used to target liposomes to specific cell-surface ligands. The ligand can also be an antibody fragment, such as Fab, or F(ab') 2 , as long as it binds efficiently to an the antigenic epitope on the target cells. Preferably, the antibody or antibody fragment recognizes an antigen that is found exclusively on the target cell. For example, certain antigens expressed specifically on macrophage cells, such as scavenger receptor A, may be exploited for the purpose of targeting antibody- ABCl liposomes directly to a macrophage cell. A number of procedures can be used to covalently attach either polyclonal or monoclonal antibodies to a liposome bilayer. Also, lipid groups can be incorporated into the lipid bilayer of the liposome in order to maintain the targeting ligand in stable association with the liposomal bilayer. In addition, various linking groups can be used for joining the lipid chains to the targeting ligand. Studies presented herein showed that ligands for nuclear receptors were able to effect an increase in cholesterol efflux. Accordingly, in another embodiment, the present invention provides a method suitable for increasing cholesterol efflux from cells in a mammalian subject comprising the step of administering to the mammalian subject at least one ligand for a nuclear receptor in an amount sufficient to increase cholesterol efflux from said cells. A pharmaceutical composition comprising a ligand for a nuclear receptor can prepared and administered using the above-described methods for formulating and administering pharmaceutical compositions. A sufficient amount of a nuclear receptor ligand is the amount of ligand that increases cholesterol efflux. Such amount can be determined by measuring the cholesterol efflux before and after administration of the ligand at various dosages and determining the dose that effects an increase in cholesterol efflux. The cholesterol efflux can be measured using assays previously described. Nuclear receptors are ligand-activated transcription factors that play a critical role in vertebrate development and adult physiology by transducing the effects of small, lipophilic hormones into transcriptional responses. Several families of nuclear receptors exist, including peroxisome proliferator-activated receptors (PPARs), liver X receptors (LXR), retinoid X receptor (RXR), the farnesoid X receptor (FXR), and the steroid and xenobiotic receptor (SXR). The PPAR family comprises the three closely related gene products PPARα, PPARγ, and PPARβ/δ. PPARα has been implicated as a key regulator of intra- and extracellular lipid metabolism. When bound to fatty acids, PPARα stimulates the proliferation of peroxisomes and induces the synthesis of several enzymes involved in the β-oxidation of fatty acids. The PPARα receptor is also the molecular target for the fibrates, drugs that are prescribed for the reduction of high triglyceride levels (Isseman et al., Nature, 347, 645 (1990)). Fibrates act as PPAR ligands to regulate the transcription of a large number of genes that affect lipoprotein and fatty acid metabolism. In addition, PPARγ ligands, such as compounds belonging to the class of thiazolidinediones compounds, have been shown to increase HDL and reduce triglyceride levels in humans. LXR is an oxysterol receptor that regulates the catabolism of excess cholesterol. LXRα has been shown to bind as a heterodimer with RXR to a DNA response element in the CYP7a gene, which encodes the enzyme responsible for the rate-limiting step in the conversion of cholesterol to bile acids. Studies have shown that mice lacking LXRα accumulate enormous amounts of cholesterol esters in their livers when fed a cholesterol-rich diet, due to their inability to increase CYP7a gene transcription in response to dietary cholesterol (Peet et al, Cell, 93: 693 (1998)). Further studies with LXRα null mice show that LXRα is also involved in the regulation of several other genes that parti cpate in cholesterol and fatty acid homeostasis. The biological role of a closely related nuclear receptor, LXRβ, which is expressed in several tissues and is activated by the same oxysterols as LXRα, remains unclear (Song et al., Proc. Natl Acad. Sci., 91 : 10809 (1994); Seol, Mol. EndocrinoL, 9: 72 (1995)). FXR, which is evolutionarily related to LXRα, is also involved in cholesterol homeostasis. Like LXRα, FXR functions as a heterodimer with the RXR receptor (Schwartz et al., Curr. Opin. Lipidol, 9: 113 (1998); Vlahcevic et al., Gastroenterology, 1 13: 1949 (1997)). FXR is activated by the synthetic retinoid TTNPB and superphysiological concentrations of all- trans retinoic acid (Zavacki et al., Proc. Natl. Acad. Sci., 94: 7909 (1997)). Recent studies indicate that FXR is a nuclear bile acid receptor. First, FXR is abundantly expressed in tissues through which bile acids circulate, including the liver, intestine, and kidney (Seol et al., Mol Endocrinol, 9: 72 (1995); Forman et al., Cell, 81 : 687 (1995)). Also, FXR has recently been shown to serve as a receptor for physiological concentrations of several bile salts, among which chenodeoxycholic acid (CDCA) is the most potent (Kliewer et al., Science, 284: 757-284 (1999); Makishima et al., Science, 284: 1362-1363 (1999)). CDCA is known to regulate the expression of several genes that participate in bile salt homeostasis, including those encoding CYP7a and the intestinal bile acid-binding protein. As described in detail in Example 13, ligands for LXR, RXR, and PPAR nuclear receptors were shown to increase apoAI-induced cholesterol efflux in cholesterol-loaded mouse macrophage cells. For example, administration of 9 c/s-retinoic acid (30 ng/ml) produced approximately a 3-fold increase in apoAI-induced cholesterol efflux in these cells. Similarly, administration of 22(R)-hydroxy cholesterol (5 μg/ml) produced approximately a 3-fold increase in apoAI-induced cholesterol efflux. Cells that received fenfibrate (3 μg/ml) produced an approximate 2-fold increase in cholesterol efflux. These results indicate that nuclear receptors may be modulated to increase the rate of apolipoprotein-mediated cholesterol efflux from macrophages. Furthermore, as described in Example 13. 9-c/s-RA mediated cholesterol efflux from macrophage cells in a dose-dependent manner. Other nuclear receptor activators, such as bezafibrate, were shown to increase cholesterol efflux (data not shown). Accordingly, in the method for increasing cholesterol efflux by administering a nuclear receptor ligand, the ligand is preferably selected from the group consisting of LXR, RXR, PPAR, FXR, and SXR nuclear receptor ligands. In the prefeoed embodiment wherein a LXR ligand is used to increase cholesterol efflux, the ligand is more preferably selected from the group consisting of 20(S) hydroxycholesterol, 22(R) hydroxycholesterol, 24-hydroxycholesterol, 25- hydroxycholesterol, and 24(S), 25 epoxycholesterol LXR ligands. Most preferably, the LXR ligand is 20(S) hydroxycholesterol. In the prefeoed embodiment wherein a RXR ligand is used to increase cholesterol efflux, the ligand is more preferably selected from the group consisting of 9-cis retinoic acid, retinol, retinal, all-trans retinoic acid, 13-cis retinoic acid, acitretin, fenretinide, etretinate, CD 495, CD564, TTNN, TTN PB, TTAB, LGD 1069. Most preferably, the RXR ligand is 9-cis retinoic acid. In another preferred embodiment wherein a PPAR ligand is used to increase cholesterol efflux, the ligand is preferably selected from the class of thiazolidinedione compounds. In another prefeoed embodiment, more than one nuclear receptor ligand is administered to the mammalian subject to increase cholesterol efflux. Preferably, when two or more nuclear receptor ligands are administered to a subject, the ligands are an LXR and an RXR ligand. More preferably, the nuclear receptor ligands are 20(S) hydroxycholesterol and 9-cis retinoic acid. In still another embodiment, the present invention provides a method suitable for increasing cholesterol efflux from cells in a mammalian subject comprising the step of administering to the mammalian subject an eicosanoid in an amount sufficient to increase cholesterol efflux. A pharmaceutical composition comprising an eicosanoid can be prepared and administered using the above-described methods for formulating and administering pharmaceutical compositions. A sufficient amount of eicosanoid is the amount that increases cholesterol efflux. Such amount can be determined by measuring the cholesterol efflux before and after administration of the eicosanoid at various dosages and determining the dose that effects an increase in cholesterol efflux. The cholesterol efflux can be measured using assays and methods previously described. As described in Example 14, eicosanoids were shown to increase apoAI-induced cholesterol efflux in cholesterol-loaded mouse macrophage cells. For example, administration of PGI2 (25 nm) produced approximately a 2-fold increase in apoAI-induced cholesterol efflux in these cells. Likewise, administration of PGE1 (25 nM) produced approximately a 3-fold increase in apoAI-induced cholesterol efflux. These results demonstrate that eicoasnoids can increase the rate of apolipoprotein-mediated cholesterol efflux from macrophages. Accordingly, in a prefeoed embodiment, the eicosanoid is selected from the group consisting of prostaglandin E2, prostacyclin (prostaglandin 12), and prostaglandin J2 eicosanoids. Methods and Compounds for Increasing ABCl Expression/ Activity Given that ABCl functions to promote cholesterol efflux, one way to increase cholesterol efflux is to increase the cellular expression of ABC 1. Accordingly, the present invention also provides methods suitable for increasing cholesterol efflux from cells in a mammalian subject by administering to the mammalian subject a therapeutic amount of a compound that increases the expression of ABCl in said cells. A therapeutic amount of compound is the amount of compound that increases ABCl expression. Such amount can be determined by measuring the gene expression of ABCl before and after administration of the compound at various dosages and determining the dose that effects an increase in ABCl gene expression. The ABCl gene expression can be measured by obtaining a blood sample from the subject, separating the monocyte population, and determining the concentration of ABCl mRNA using methods known in the art and described herein, such as RT-PCR. In one prefeoed embodiment, the method comprises administering a cAMP analogue to increase the gene expression of ABCl . As shown in Figure 8, cAMP increases the expression of ABCl mRNA in normal fibroblast cells approximately 10-fold. Preferably, the cAMP analogue is selected from the group consisting of 8-bromo cAMP, N6-benzoyl cAMP, and 8-thiomethyl cAMP. In another preferred embodiment, the method comprises administering a compound that increases the synthesis of cAMP to increase the gene expression of ABCl . Preferably, the compound is forskolin. In yet another prefeoed embodiment, the method comprises administering a compound that inhibits the breakdown of cAMP to increase the gene expression of ABCl. An example of such a compound is a phosphodiesterase inhibitor. Preferably, the phosphodiesterase inhibitor is selected from the group consisting of rolipram, theophylline, 3- isobutyl-1-methylxanthine, R020-1724, vinpocetine, zaprinast, dipyridamole, milrinone, amrinone, pimobendan, cilostamide, enoximone, peroximone, and vesnarinone phosphodiesterase inhibitors. In another prefeoed embodiment, the method comprises administering to the mammalian subject a ligand for a nuclear receptor in an amount sufficient to increase the gene expression of ABC 1. As described in Example 17 and shown in Figure 12, ligands for nuclear receptors can up-regulate the gene expression of ABC 1. Transfection studies using pAPR 1 , which contains a luciferase reporter gene under the control of the ABCl promoter, showed that the ABCl promoter was activated in the presence of ligands for LXR and RXR nuclear receptors. Specifically, macrophage cells transfected with pAPRl produced a 19-fold increase in luciferase reporter activity in the presence of 20 OH-chol, a 16-fold increase in luciferase activity in the presence of 9-cis RA, and a 280-fold increase in luciferase activity in the presence of both ligands compared with EtOH control. The results indicate that both sterols and retinoids elicit a strong transcription response from the ABC 1 promoter. Further, there is an apparent synergistic effect between the two classes of compounds, as can be seen by the dramatic increase in luciferase activity found in cells treated with both ligands. In accordance with the inventive method, preferably, the ligand is selected from the group consisting of LXR, RXR, PPAR, FXR, and SXR ligands. In addition to increasing cellular levels of ABC 1 protein, reverse cholesterol transport can be promoted by enhancing the activity of ABCl protein. Thus, in another embodiment, the present invention provides a method suitable for increasing cholesterol efflux from cells in a mammalian subject comprising the step of administering to the mammalian subject a therapeutic amount of a compound that increases ABC 1 activity in an amount sufficient to increase cholesterol efflux. A pharmaceutical composition comprising such a compound can prepared and administered using the above-described methods for formulating and administering pharmaceutical compositions. A therapeutic amount of compound is the amount of compound that increases cholesterol efflux. Such amount can be determined by measuring the cholesterol efflux before and after administration of the compound at various dosages and determining the dose that effects an increase in cholesterol efflux using methods previously described. To determine whether an increase in cholesterol efflux is due to an increase in ABCl activity, the amount of ABCl protein present in a cell sample before and after administration of the compound is determined, using methods described herein (see Example 1 1). For both measurements (i.e. pre- and post-administration of the compound), the amount of cholesterol efflux activity is divided by the concentration of ABCl protein to determine the amount of cholesterol activity found in a standard concentration of ABCl protein. An observed increase in cholesterol activity standardized to protein concentration indicates that the increase is due to an increase in ABCl activity. Methods for Identifying Therapeutic Compounds Another aspect of the present invention relates to methods for screening a compound to determine whether the compound modulates (i.e., up-regulates or down-regulates) the gene expression of ABC 1. Such compounds may be useful in the development of therapeutic compounds that increase ABCl expression and thereby promote cholesterol efflux and raise blood levels of HDL-cholesterol. Accordingly, methods for identifying compounds that may be useful in the treatment of cardiovascular disease are provided. In one embodiment, the present invention provides a method for screening a test compound for ABCl expression modulating activity comprising the steps of: (a) operatively linking a reporter cDNA with an expression modulating portion of the mammalian ABCl gene to produce a recombinant reporter construct; (b) transfecting the recombinant reporter construct into a population of host cells; (c) assaying the level of reporter gene expression in a sample of the transfected host cells; (d) contacting the transfected host cells with the test compound being screened; (e) assaying the level of reporter gene expression in a sample of the transfected host cells after contact with the test compound; and (f) comparing the relative change in the level of reporter gene expression caused by exposure to the test compound, thereby determining the ABCl expression modulating activity. First, a recombinant reporter construct comprising a heterologous reporter gene operatively linked to an expression modulating portion of the ABCl gene is constructed. The ABCl expression modulating polynucleotide and reporter gene can be inserted into a vector using well-known ligation and cloning techniques, such as those described herein and in standard laboratory manuals, including Davis et al., Basic Methods in Molecular Biology (1986); Sambrook et al., Molecular Cloning: A Laboratory Manual, 2 nd Ed., (Cold Spring Harbor Press, Cold Spring Harbor, N.Y. (1989)); and Ausubel et al. eds., Current Protocols in Molecular Biology, (Wiley and Sons (1994)). Alternatively, the ABCl expression modulating polynucleotide can be inserted into a commercially available reporter construct, such as those previously described. Any vector suitable for ABCl polynucleotide and reporter gene insertion can be used. The chosen vector should be functional in the particular host cell employed. Preferably, the vector is compatible with mammalian host cells. Preferably, the expression modulating portion of the ABCl gene is the 5' flanking region of ABCl, containing ABCl promoter activity. In one prefeoed embodiment, the expression modulating portion of the ABCl gene comprises SEQ ID NO: 3. In another prefeoed embodiment, the expression modulating portion of the ABCl gene comprises nucleotides 1- 1532, 1080-1643, 1 181-1643, 1292-1643, 1394-1643, or 1394-1532 of SEQ ID NO: 3. Also, preferably the heterologous reporter is selected from the group consisting of polynucleotides that encode the luciferase, β-galactosidase, chloramphenicol acetyl transferase, and green fluorescent proteins. More preferably, the heterologous reporter is a polynucleotide that encodes the luciferase protein. In a particularly prefeoed embodiment, the recombinant reporter construct is pAPRl, shown in Figure 1 1. Next, the recombinant reporter construct is transfected into a population of host cells. The recombinant reporter construct can be introduced into the host cells using any of the previously described transfection methods, as well as the methods described in Examples 8 and 15. For example, the reporter construct can be transfected using calcium phosphate transfection, DEAE-dextran mediated transfection, cationic lipid-mediated transfection, electroporation, transduction, infection, or any of the other known and described methods (see, e.g., Davis et al., Basic Methods in Molecular Biology (1986)). The host cell can be any cell that, when cultured under appropriate conditions, synthesizes a reporter polypeptide, which can be subsequently measured. Preferably, the host cell is a mammalian host cell. More preferably, the mammalian host cell is a macrophage, fibroblast, hepatic, or intestinal cell. Most preferably, the host cell is selected from the group consisting of RAW 264.7 cells, Thp-1 cells, and HepG2 cells. The concentration of reporter construct and duration of the transfection can vary, depending on the transfection method and the type and concentration of host cell used. Determination of the appropriate concentration of reporter construct and transfection time is well within the skill of the ordinary artisan. Following transfection, a sample of transfected host cells that was not exposed to the test compound is assayed to determine the level of reporter gene expression. The level of reporter gene expression found in the sample of unexposed transfected host cells provides a control measurement. The transfected host cells are lysed and the level of reporter gene expression is assayed using any of the methods well-known in the art. The assays used to measure the level of reporter gene expression differ, depending on the reporter construct used in the transfections. For example, if a luciferase reporter construct is used, the luciferase activity of the cell lysate is measured as light units using a luminometer, as described in Example 15. A different sample of the transfected host cells is contacted with the test compound being screened and the level of reporter gene expression found in these cells is subsequently measured. Preferably, the transfected host cells are contacted with the test compound for about 4-48 hours. More preferably, the transfected host cells are contacted with the test compound for about 8-36 hours. Even more preferably, the transfected host cells are contacted with the test compound for about 24 hours. The same assay used to measure the level of reporter gene expression in unexposed (control) cells should be used to measure the level of reporter gene expression in the cells exposed to the test compound. Finally, the level of reporter gene expression found in unexposed control cells is compared with the level of reporter gene expression found in cells exposed to the test compound to determine whether the test compound has ABCl expression modulating activity. If the level of gene expression in both cell samples are the same or about the same, the test compound does not modulate ABCl gene expression. A higher level of reporter gene expression in cells exposed to the test compound relative to the level of reporter gene expression found in control cells, indicates that the test compound up-regulates the gene expression of ABC 1. A lower level of reporter gene expression in cells exposed to the test compound relative to the level of reporter gene expression found in control cells, indicates that the test compound down-regulates the gene expression of ABC 1. Another aspect of the present invention relates to methods for screening a test compound to determine whether the compound promotes ABCl -mediated cholesterol efflux. Such method comprises: (a) assaying the level of cholesterol efflux in a sample of mammalian cells maintained in culture to determine a control level of cholesterol efflux, (b) contacting the mammalian cells with the test compound being screened; (c) assaying the level of cholesterol efflux in a sample of cells after contact with the test compound: and (d) assaying the level of ABCl -dependent cholesterol efflux in a sample of cells after contact with the test compound, thereby determining whether the test compound promotes ABCl -mediated cholesterol efflux from cells in culture. The level of cholesterol efflux in a sample of cultured cells can be determined using methods known in the art and described herein (see Example 1). Any mammalian cells that can be maintained in culture can be used to measure cholesterol efflux. The cells can be derived from primary cultures or from immortalized cell lines. For convenience, cells can be immortalized by transfecting them with amphotropic retroviruses containing vectors with inserts of human papillomavirus 16, oncogenes E6 and E7, and a selectable marker gene, as described in Example 1. Preferably, the cultured cells are fibroblast, macrophage, hepatic, or intestinal cells. More preferably, the cultured cells are RAW 264.7 cells. The level of cholesterol efflux in a sample of cells that has not been contacted with the test compound is measured to obtain a control level of cholesterol efflux. In addition, the level of cholesterol efflux in a sample of mammalian cells that has been contacted with the test compound is measured to determine the amount of cholesterol efflux affected by the test compound. Also, the level of ABCl -mediated cholesterol efflux in a sample of mammalian cells that has been contacted with the test compound is measured to determine the amount of ABCl - mediated cholesterol efflux affected by the test compound. Preferably, the cells are contacted with the test compound for about 8-24 hours before cholesterol efflux or ABCl -mediated cholesterol efflux is assayed. The level of ABCl -mediated cholesterol efflux can be assayed using an anti-ABC 1 antibody that, upon binding, inhibits the activity of ABCl . Alternatively, the level of ABCl -mediated cholesterol efflux can be assayed using an anti-sense ABCl polynucleotide that inhibits the expression of ABCl . For example, the level of ABCl -mediated cholesterol efflux can be assayed using the anti-sense ABCl polynucleotide comprising SEQ ID NO: 57 (see Example 7). The cells should be contacted with the anti-ABCl antibody or anti- sense ABCl polynucleotide at the same time and for the same duration that it is contacted with the test compound. If the level of control cholesterol efflux is the same or about the same as the level of cholesterol efflux found in cells contacted with the test compound, the compound does not promote cholesterol efflux. An increase in the level of cholesterol efflux found in cells contacted with test compound over the control level of cholesterol efflux indicates the amount of cholesterol efflux promoted by the test compound. The difference between the cholesterol efflux found in cells contacted with test compound alone and the cholesterol efflux found in cells contacted with test compound and anti-ABCl antibody or anti-sense ABCl polynucleotide indicates the amount of cholesterol efflux mediated through ABCl . For example, if control level of cholesterol efflux is 1.0 and the level of cholesterol efflux found in cells contacted with test compound is 1.1, the test compound promotes cholesterol efflux by 10%. If the cholesterol efflux found in cells contacted with test compound and anti-ABCl antibody or anti-sense ABCl polynucleotide is 1.0, the increase in cholesterol efflux caused by the test compound is entirely ABCl -mediated. Methods for Detecting Susceptibility to Coronary Heart Disease The present invention also relates to methods for detecting the comparative level of ABCl gene or protein expression in a mammalian subject, including a human subject. Given the role of ABCl in cholesterol efflux, the determination of a decreased level of ABCl gene or protein expression in a mammalian subject relative to a pre-determined standard level of ABCl gene or protein expression can be used to indicate a susceptibility to coronary heart disease in the subject. Accordingly, the present invention provides a method for detecting the comparative level of ABCl gene expression in a mammalian subject comprising the steps of: (a) obtaining a test cell sample from the mammalian subject; (b) assaying the level of ABCl mRNA expression in the test cell sample; and (c) comparing the level of ABCl mRNA expression in the test cell sample with a pre-determined standard level of ABCl mRNA expression, thereby detecting the comparative level of ABCl gene expression in the mammalian subject. A test cell sample is first obtained from a mammalian subject, including a human subject. The test cell sample can be a blood sample, wherein the monocyte population has been enriched. Monocytes can be enriched using well-known cell separation procedures based on, for example, cell size, cell density or cell affinity. Next, the level of ABCl mRNA expression in the test cell sample is assayed. The level of ABCl mRNA expression can be assayed using any of the well- known methods for mRNA preparation and detection, including the methods described herein at Examples 2 and 9. For example, the concentration of ABCl mRNA can be determined by reverse transcription polymerase chain reaction, northern blot analysis, or RNAse protection assay. The ABCl mRNA concentration should be standardized to the concentration of total mRNA found in the test cell sample. Finally, the ABCl expression in the test cell sample is compared with a pre-determined standard level of ABCl mRNA expression. A pre-determined standard level of ABCl mRNA expression can be obtained by determining the average concentrations and distribution of ABCl mRNA found in cell samples taken from a representative population of mammalian subjects, wherein the mammalian subjects are the same species as the subject from which the test cell sample was obtained, and wherein the mammalian subjects do not have coronary heart disease, Tangier disease, or other disease associated with low HDL-cholesterol and are considered to have cholesterol efflux activity within a normal range (i.e., as indicated by an HDL-cholesterol level within a normal range). The determination of a decreased level of ABCl mRNA expression in the test cell sample of a mammalian subject relative to the pre-determined standard level of ABCl mRNA expression can be used to indicate a susceptibility to coronary heart disease in the mammalian subject. Likewise, the detection of a decreased level of ABCl protein can be used to indicate decreased capacity for cholesterol efflux and a susceptibility to coronary heart disease. Accordingly, another embodiment of the present invention provides a method for detecting the comparative level of ABCl protein in a mammalian subject. Such method comprises the steps of: (a) obtaining a test cell sample from the mammalian subject; (b) assaying the amount of ABCl protein in the test cell sample; and (c) comparing the amount of ABCl protein in the test cell sample with a pre-determined standard amount of ABCl protein, thereby detecting the comparative level of ABCl protein in the mammalian subject. The amount of ABCl protein can be assayed using any of the well-known methods of measuring protein. Preferably, the amount of ABCl protein is measured using an immunoassay. In one embodiment, the amount of ABCl protein is determined by (a) contacting the cell sample with a population of anti-ABCl antibodies and (b) detecting the anti-ABCl antibodies associated with the cell sample. For example, the ABCl protein can be contacted with an antiserum raised against a synthetic peptide corresponding to KNQTWDAVLTSFLQDEKVKES located at the C-terminus, as described in Example 1 1. The anti-ABCl antibodies can be detected using several methods known in the art, including, for example, western blotting, immunoprecipitation, and FACS, wherein the detection can be accomplished using radioactive, colorometric, or fluorescent labeling. One preferred method for measuring the amount of ABCl protein in a cell sample is immunoprecipitation, wherein biotinylated ABCl proteins are contacted with anti- ABCl antibody and the bound anti-ABCl antibody is detected using streptavidin horse radish peroxidase. The amount ABCl protein in the test cell sample is compared with a pre-determined standard amount of ABCl protein. A pre-determined standard amount of ABCl protein can be obtained by determining the average concentration of ABCl protein found in cell samples taken from a population of mammalian subjects, wherein the mammalian subjects are the same species as the subject from which the test cell sample was obtained, and wherein the mammalian subjects do not have coronary heart disease, Tangier disease, or other disease associated with low HDL- cholesterol and are considered to have cholesterol efflux activity within a normal range (i.e., as indicated by an HDL-cholesterol level within a normal range). ABCl Antibodies As used herein, the term "antibody" (Ab) or "monoclonal antibody" (Mab) is meant to include intact molecules as well as antibody fragments (such as, for example, Fab and F(ab')2 fragments) which are capable of specifically binding to protein. Fab and F(ab')2 fragments lack the Fc fragment of intact antibody, clear more rapidly from the circulation, and may have less non-specific tissue binding than an intact antibody (Wahl et al., J. Nucl Med., 24:316-325 (1983)). Thus, these fragments are preferred, as well as the products of a FAB or other immunoglobulin expression library. Moreover, antibodies of the present invention include chimeric, single chain, and humanized antibodies. Additional embodiments include chimeric antibodies, e.g., humanized versions of murine monoclonal antibodies. Such humanized antibodies may be prepared by known techniques, and offer the advantage of reduced immunogenicity when the antibodies are administered to humans. In one embodiment, a humanized monoclonal antibody comprises the variable region of a murine antibody (or just the antigen binding site thereof) and a constant region derived from a human antibody. Alternatively, a humanized antibody fragment may comprise the antigen binding site of a murine monoclonal antibody and a variable region fragment (lacking the antigen-binding site) derived from a human antibody. Procedures for the production of chimeric and further engineered monoclonal antibodies include those described in Riechmann et al. (Nature 332:323, 1988), Liu et al. (PNAS 84:3439, 1987), Larrick et al. (Bio/Technology 7:934, 1989), and Winter and Harris (TIPS 4: 139, May, 1993). One method for producing an antibody comprises immunizing a non-human animal, such as a transgenic mouse, with a polypeptide translated from a polynucleotide comprising SEQ ID NO: 1, a polynucleotide comprising nucleotides 291-7074 of SEQ ID NO: 1, or a polynucleotide comprising a nucleotide sequence that has at least 90% identity with a polynucleotide comprising SEQ ID NO: 1, whereby antibodies directed against the polypeptide translated from the described polynucleotides are generated in said animal. Procedures have been developed for generating human antibodies in non-human animals. The antibodies may be partially human, or preferably completely human. Non-human animals (such as transgenic mice) into which genetic material encoding one or more human immunoglobulin chains has been introduced may be employed. Such transgenic mice may be genetically altered in a variety of ways. The genetic manipulation may result in human immunoglobulin polypeptide chains replacing endogenous immunoglobulin chains in at least some (preferably virtually all) antibodies produced by the animal upon immunization. Antibodies produced by immunizing transgenic animals with a polypeptide translated from any of the described polynucleotides are provided herein. Mice in which one or more endogenous immunoglobulin genes are inactivated by various means have been prepared. Human immunoglobulin genes have been introduced into the mice to replace the inactivated mouse genes. Antibodies produced in the animals incorporate human immunoglobulin polypeptide chains encoded by the human genetic material introduced into the animal. Examples of techniques for production and use of such transgenic animals are described in U.S. Patents 5,814,318, 5,569,825, and 5,545,806, which are incorporated by reference herein. Monoclonal antibodies may be produced by conventional procedures, e.g., by immortalizing spleen cells harvested from the transgenic animal after completion of the immunization schedule. The spleen cells may be fused with myeloma cells to produce hybridomas, by conventional procedures. A method for producing a hybridoma cell line comprises immunizing such a transgenic animal with a immunogen comprising at least seven contiguous amino acid residues of a polypeptide translated from one of the described polynucleotides; harvesting spleen cells from the immunized animal; fusing the harvested spleen cells to a myeloma cell line, thereby generating hybridoma cells; and identifying a hybridoma cell line that produces a monoclonal antibody that binds a polypeptide translated from one of the described polynucleotides. Such hybridoma cell lines, and monoclonal antibodies produced therefrom, are encompassed by the present invention. Monoclonal antibodies secreted by the hybridoma cell line are purified by conventional techniques. The antibodies, upon specific binding to an ABCl polypeptide, may inhibit the activity of the ABC 1 polypeptide. Preferably, the antibody, upon binding, inhibits the cholesterol transport activity of the ABCl polypeptide. Such antibodies can be made by immunizing a non-human animal with a polypeptide cooesponding to the region essential for cholesterol transport. The antibody can be tested to determine whether it inhibits cholesterol efflux using any of the described cholesterol efflux assays. Such inactivating antibodies can be employed in an in vitro assay, such as any of the cholesterol efflux assays described herein, to determine whether a test compound promotes ABCl -mediated cholesterol efflux. The inactivating antibodies can also be used in in vitro assays to detect the comparative level of ABCl protein in the cells of a mammalian subject. The inactivating antibodies are also useful in kits suitable for screening a compound to determine whether the compound modulates ABCl -dependent cholesterol efflux. Kits for Identifying Therapeutic Compounds The present invention also includes a kit suitable for screening a compound to determine the ABCl expression modulating activity of a compound. The kit includes, in an amount sufficient to perform at least one assay, a recombinant reporter construct comprising a reporter cDNA operatively linked to an expression modulating portion of the mammalian ABCl gene, as a separately packaged reagent. Instructions for use of the packaged reagent(s) are also typically included. The expression modulating portion of the mammalian ABCl gene comprises the 5' flanking sequence. In one prefeoed embodiment, the expression modulating portion of the mammalian ABCl gene comprises SEQ ID NO: 3. In other prefeoed embodiments, the expression modulating portion of the mammalian ABCl gene comprises nucleotides 1-1532, 1080-1643, 1 181-1643, 1292-1643, 1394-1643, or 1394-1532 of SEQ ID NO: 3. The reporter cDNA can be any suitable reporter gene, including In a particularly preferred embodiment, the recombinant reporter construct is pAPRl . In another embodiment, the kit further comprises means for detecting the reporter protein. Thus, the kit comprises reagents, such as buffers and substrates, used for reporter protein detection. As used herein, the term "package" refers to a solid matrix or material such as glass, plastic (e.g., polyethylene, polypropylene or polycarbonate), paper, foil and the like capable of holding within fixed limits a recombinant vector of the present invention. Thus, for example, a package can be a glass vial used to contain milligram quantities of a contemplated vector. "Instructions for use" typically include a tangible expression describing the reagent concentration or at least one assay method parameter such as the relative amounts of reagent and sample to be admixed, maintenance time periods for reagent/ sample admixtures, temperature, buffer conditions and the like. In addition, the present invention also includes a kit suitable for screening a compound to determine whether the compound modulates ABCl -dependent cholesterol efflux. In one embodiment, the kit includes, in an amount sufficient to perform at least one assay, an inactivating anti-ABCl antibody, as a separately packaged reagent. Instructions for use of the packaged reagent(s) are also typically included. In another embodiment, the kit includes an antisense ABCl oligonucleotide in an amount sufficient for at least one assay and instructions for use. Preferably, the antisense ABCl oligonucleotide comprises SEQ ID NO: 53. Microarrays It will be appreciated that DNA microaoay technology can be utilized in accordance with the present invention. DNA microaoays are miniature, high-density aoays of nucleic acids positioned on a solid support, such as glass. Each cell or element within the aoay contains numerous copies of a single nucleic acid species that acts as a target for hybridization with a complementary nucleic acid sequence (e.g., mRNA). In expression profiling using DNA microaoay technology, mRNA is first extracted from a cell or tissue sample and then converted enzymatically to fluorescently labeled cDNA. This material is hybridized to the microaoay and unbound cDNA is removed by washing. The expression of discrete genes represented on the aoay is then visualized by quantitating the amount of labeled cDNA that is specifically bound to each target nucleic acid molecule. In this way, the expression of thousands of genes can be quantitated in a high throughput, parallel manner from a single sample of biological material. This high throughput expression profiling has a broad range of applications with respect to the ABCl molecules of the invention, including, but not limited to: the identification and validation of ABCl disease-related genes as targets for therapeutics; molecular toxicology of related ABCl molecules and inhibitors thereof; stratification of populations and generation of suoogate markers for clinical trials; and enhancing related ABCl polypeptide small molecule drug discovery by aiding in the identification of selective compounds in high throughput screens. As discussed herein at Example 2, a method has been developed that uses samples of RNA derived from cells of an individual with a genetic abnormality and compares them to the RNA from a normal individual. Historically, identification of the cause of inherited diseases resulted from years of biochemical analysis or, more recently, from years of gene mapping and positional cloning to identify the suspect gene within an interval of millions of base pairs which had been shown to be closely linked to the defect in inheritance studies (linkage analysis). The use of multigene expression analysis, most notably via "gene chips" can revolutionize the pace of such discovery. Comparing the expression of samples of RNA derived from cells from an abnormal individual with a genetic disease versus RNA from an normal individual can quickly reveal genes whose cooesponding mRNA is missing, severely undeoepresented or severely oveoepresented in the abnormal diseased cell. The term "individual" and used herein refers to any living organism that has RNA such as, for example, mammals, plants. This method is preferably used to identify the source of human genetic abnormalities. More preferably, the method is useful for detecting genetic sources of human cardiac and cardiovascular disorders such as identifying ABCl as the genetic defect in Tangier's disease. The term "abnoonality" as it is used herein refers to genetic differences that causes a physiological deviation in small number of individuals in a species in comparison to the majority of individuals of the species. The abnormality may be a positive abnormality or a negative abnormality. For example, a positive plant abnormality would be a genetic difference that causes some individual plants to be drought resistant in comparison to the other individuals in the species. A negative abnormality would be one that causes an individual in the same species of plant to me more prone to drought damage than a normal plant. The method can best be described by reference to our investigation into the genetic cause of Tangier's disease. We began our investigation by using RNA from cells cultured from an individual with Tangier disease to probe microaoays containing nearly 60,000 normal human genes, and we were able to use the probe results to identify ABCl as the defective gene in this monogenic disease in which patients have near zero levels of circulating high density lipoprotein (HDL) and an increased risk of heart disease. It is not necessary that the defective gene results in a zero level of detectable mRNA signal in such an experiment. In this successful example, roughly 175 out of the 58,800 probes on the micro-aoay were more than 2.5 fold underexpressed in the Tangier disease RNA versus normal. Several additional steps may be taken to confirm the identity of the culprit gene. They include repeating such a micro-aoay probe with an unrelated individual with Tangier disease, determining the chromosome map location of each gene to compare with a reported large genetic interval that was linked to the disease, consideration of the likely function of the candidate proteins and their homologs, biochemical tests, and sequencing the best candidate gene in patients to find mutations. In these ways, gene expression micro-aoay analysis can lead to the identification of inherited genetic defects such as the identification of ABCl as the defect in Tangier disease A further utility in this method is that other genes that are either under- or over-expressed in the disease sample vs. normal should include those that are differentially regulated in consequence of the genetic defect in the patient, either as compensatory responses or as contributors to the disease pathology. This could provide identification of other proteins in the relevant biological pathways that may be amenable to drug development and help elucidate the pathology of the disease, with implications for treatment and diagnosis. In the cases where a gene deletion or other mutation causes complete absence of mRNA, as observed in many examples of thalassemia (globin gene defects) and other genetic diseases, gene expression analysis of disease versus normal samples can lead to the identification of the missing gene in a more straightforward manner. Although in these examples, the gene expression aoay that was probed with RNA samples was of the type in which probe samples were cDNAs aoayed on microscope slides, alternative aoay technologies would suffice. These would include, but not limited to those which aoay DNA samples on filter membranes or use oligonucleotide probes synthesized on 'gene chips" by photolithography. Generally, the term microaoay refers to an aoay of distinct oligonucleotides synthesized on a substrate, such as paper, nylon or other type of membrane, filter, chip, glass slide, or any other suitable solid support. Microaoays may be prepared, used, and analyzed using methods known in the art. (See, e.g., Brennan, T. M. et al. ( 1995) U.S. Pat. No. 5,474,796; PCT application W095/251 1 16; Shalon, D. et al. ( 1995) PCT application W095/35505; and U.S. Pat. No. 5,605,662 the specifications of each of which are incorporated herein by reference) A chemical coupling procedure and an ink jet device can be used to synthesize aoay elements on the surface of the substrate. An aoay analogous to a dot or slot blot may also be used to aoange and link elements to the surface of a substrate using thermal, UV, chemical, or mechanical bonding procedures. A typical aoay may be produced by hand or using available methods and machines and contain any appropriate number of elements. After hybridization, nonhybridized probes are removed and a scanner used to determine the levels and patterns of fluorescence. The degree of complimentrity and the relative abundance of each probe which hybridizes to an element on the microaoay may be assessed through analysis of the scanned images. Full-length cDNAs, Expressed Sequence Tags (ESTs), or fragments thereof may comprise the elements of the microaoay. Fragments suitable for hybridization can be selected using software well known in the art such as LASERGENE software (DNASTAR). Full-length cDNAs, ESTs, or fragments thereof cooesponding to the nucleotide sequences of an abnoonal individual and a noonal individual are arranged on an appropriate substrate, e.g., a glass slide. The cDNA is fixed to the slide using, e.g., UV cross-linking followed by thermal and chemical treatments and subsequent drying. (See, e.g., Schena, M. et al. (1995) Science 270:467-470; Shalon, D. et al. (1996) Genome Res. 6:639-645.) Probes, such as fluorescent probes are prepared and used for hybridization to the elements on the substrate. In order to conduct sample analysis using the microaoays, the RNA or DNA from a biological sample is made into hybridization probes. The mRNA is isolated, and cDNA is produced and used as a template to make antisense RNA (aRNA). The aRNA is amplified in the presence of fluorescent nucleotides, and labeled probes are incubated with the microaoay so that the probe sequences hybridize to complementary oligonucleotides of the microaoay. Incubation conditions are adjusted so that hybridization occurs with precise complementary matches or with various degrees of less complementarity. After removal of nonhybridized probes, a scanner is used to determine the levels and patterns of fluorescence. The scanned images are examined to determine degree of complementarity and the relative abundance of each oligonucleotide sequence on the microaoay. The biological samples may be obtained from any bodily fluids (such as blood, urine, saliva, phlegm, gastric juices, etc.), cultured cells, biopsies, or other tissue preparations. A detection system may be used to measure the absence, presence, and amount of hybridization for all of the distinct sequences simultaneously. This data may be used for large scale cooelation studies on the sequences, mutations, variants, or polymorphisms among samples. The microaoay is preferably composed of a large number of unique, single-stranded nucleic acid sequences, usually either synthetic antisense oligonucleotides or fragments of cDNAs fixed to a solid support. Microarrays may contain oligonucleotides which cover the known 5', or 3', sequence, or contain sequential oligonucleotides which cover the full length sequence; or unique oligonucleotides selected from particular areas along the length of the sequence. Polynucleotides used in the microaoay may be oligonucleotides that are specific to a gene or genes of interest in which at least a fragment of the sequence is known or that are specific to one or more unidentified cDNAs which are common to a particular cell type, developmental or disease state. In order to produce oligonucleotides to a known sequence for a microaoay, the gene of interest is examined using a computer algorithm which starts at the 5' or more preferably at the 3' end of the nucleotide sequence. The algorithm identifies oligomers of defined length that are unique to the gene, have a GC content within a range suitable for hybridization, and lack predicted secondary structure that may interfere with hybridization. The oligomers are synthesized at designated areas on a substrate using a light-directed chemical process. The substrate may be paper, nylon or other type of membrane, filter, chip, glass slide or any other suitable solid support. An aoay may be produced by hand or using available devises (slot blot or dot blot apparatus) materials and machines (including robotic instruments) and contain grids of 8 dots, 24 dots, 96 dots, 384 dots, 1536 dots or 6144 dots, or any other multiple which lends itself to the efficient use of commercially available instrumentation. Once the genetic causes of the inherited abnormality are naoowed or identified, the potentially or actual defective portions of the genes can be used as targets in a microaoay. The microaoay can be used to monitor the expression level of a large number of genes to develop and monitor the activities of potential therapies and therapeutic agents. The following examples further illustrate the present invention but should not be construed to limit the present invention in any way. EXAMPLE 1 This example demonstrates that patients with Tangier Disease (TD) have an absence of apo A-I-mediated lipid efflux. Cell Cultures: Human fibroblasts were obtained from skin explants from two normal subjects (NL1 ) and three unrelated patients with Tangier disease (TD). TD1 cells were obtained from a 53 year-old female with extremely low plasma HDL cholesterol and apo A-I levels and clinical symptoms typical of Tangier disease. TD2 cells were obtained from a 56 year-old male with clinical, morphological, and biochemical features of Tangier disease, including very low levels of plasma HDL cholesterol and apoA-I (France, et al., J. Clin. Invest., 96, 78-87 (1995)). TD3 cells were obtained from an 18 year-old male with Tangier disease who presented with orange tonsil remnants, asymmetrical motor neuropathy, plasma HDL cholesterol of 5 mg/dL, and LDL cholesterol of 16mg/dL (Lawn et al., J. Clin. Invest., 104, R25-R31 (1999)). The normal cells and TD subject cells were immortalized as described in Oram et al., J. Lipid Res., 40: 1769-1781 (1999). Briefly, the cells were transfected with amphotropic retroviruses containing vectors with inserts of human papillomavirus 16 oncogenes E6 and E7 and a neomycin resistance selectable marker. Control cells were infected with vector alone (mock- infected). Pooled cell populations were selected in the presence of G418 for two passages, after which G418 was excluded from the medium. Fibroblasts were used between the fifth and sixteenth passage (primary) or sixth and fourteenth passage (immortalized). The immortalized normal and TD cells were seeded into 16-mm wells or 35-mm dishes and grown to confluence in Dulbecco's modified Eagle's medium (DMEM) plus 10% fetal bovine serum (FBS) before experimental use. RAW 264.7 mouse monocytic cells (American Type Culture Collection, Rockville, MD) were also maintained in DMEM containing 10% FBS. Assay to Measure Lipid Effux: Apo Al-mediated efflux of cholesterol and phospholipid was assayed according to the method described in Franco, et al., J. Clin. Invest., 96: 78-87 (1995). The cultured skin fibroblasts from normal and TD subjects were labeled by growth to confluence in the presence of 0.2-0.5 μCi/ml [ 3 H] cholesterol (40-60 Ci/mmol, Amersham Corp., Arlington Heights, IL). The radioactive cholesterol was added to serum-containing growth medium when the cells were 60-80% confluent. After 3 days, the cells were washed twice with PBS/BSA and simultaneously growth-aoested and cholesterol-loaded to maximize apolipoprotein-mediated lipid efflux. This was achieved by incubating the cells for 48 hours in serum-free DMEM plus 2 mg/ml fatty acid-free bovine serum albumin (DMEM/BSA) (Sigma Chemical Co., St. Louis, MO) and 30 μg/ml non-radioactive cholesterol. RAW 264.7 cells were cholesterol-loaded through the scavenger receptor by 24-hour incubation with acetylated LDL as described in Smith et al., J. Biol Chem., 271 :30647-30655 (1996). Briefly, RAW 264.7 cells in 24-well dishes were cholesterol-loaded and labeled overnight in 0.5 ml of DMEM supplemented with 50 μl/ml 1M glucose, 10 μl/ml 200 mM glutamine and 2% BSA and with 50 μg/ml acetylated low density lipoprotein (AcLDL) and [ 3 H]-cholesterol which had been pre-incubated for 30 minutes at 37°C with AcLDL to yield a final concentration of 0.33 μCi/ml [ 3 H]- cholesterol. Cells were subsequently washed five times with PBS containing 1% BSA and incubated overnight (16-18 hours) in DMEM/BSA to allow for equilibrium of cholesterol pools. After equilibrium of cholesterol pools, cells were rinsed four times with PBS/BSA and incubated for one hour at 37°C with DMEM/BSA before the efflux incubations. Efflux medium (DMEM/BSA) containing either albumin alone (control), albumin plus HDL (40μg protein/ml), or albumin plus apo A-I (10 μg/ml, Biodesign International, Kennebunk, ME) was added and the cells were incubated for 4, 24, or 48 hours. Phospholipids were labeled by including 10 μCi/ml [ 3 H]choline (75-85 Ci/mmol, Amersham Corp.) in the DMEM/BSA overnight equilibrium medium. The radioactivity found in the culture medium was measured by scintillation counting after a 15-minute centrifugation at 12,000g. The radioactivity in the cells was measured by scintillation counting after solubilization in 0.5 ml of 0.2M NaOH (Smith et al., J. Biol. Chem., 271 :30647-30655 (1996)) or extraction in hexane:isopropanol (3:2 v/v) as described in Franc/-?, et al., J. Clin. Invest., 96, 78-87 (1995). Cells containing labeled phospholipids were extracted with 1 ml of isopropanol for 1 hour and then with hexanetisopropanol as described above. The efflux of cholesterol or phospholipid was expressed as the percentage of tritiated lipid counts in the medium over the total tritiated lipid counts recovered from the cells and medium (cpm medium / cpm (medium + lysate) x 100). As shown in Figure 1 A and C, the addition of HDL or apo A-I results in the removal of cholesterol from cholesterol-laden fibroblasts obtained from normal subjects. However, in TD cells, the ability of HDL to remove cholesterol is slightly diminished and the ability of apo A-I to remove cholesterol is completely absent. Figure 1 shows that normal and TD fibroblast cells release about 3-4% of the cellular [ 3 H]-cholesterol into the medium during 48-hour incubation with albumin. Addition of HDL to the albumin medium increased the efflux of [ 3 H]-cholesterol from both normal and TD fibroblasts, although to a lesser extent with TD cells (Figure IB, D). Addition of apo A-I promoted the efflux of [ 3 H]-cholesterol from normal fibroblasts (Figure 1A, C), but had little or no effect on [ 3 H]-cholesterol efflux from TD fibroblasts (Figure IB, D). EXAMPLE 2 This example demonstrates that 175 genes show at least 2.5-fold decreased expression and 375 genes show at least 2.5-fold increased expression in TD cells compared with normal cells. The differential gene expression was determined using gene-expression microaoay (GEM) analysis of cDNAs from normal individuals (non-TD) and from patients with TD. Cell Cultures: The immortalized cell cultures obtained from normal individuals and TD patients described in Example 1 were used. Confluent cultures were maintained in DMEM BSA and supplemented for 24 hours with ImM 8-Bromo cyclic adenosine monophosphate (8-Br- cAMP, Sigma Chemical Co., St. Louis, MO). mRNA Extraction and cDNA Synthesis: mRNA from both normal and TD fibroblast cells was prepared from total RNA extracted from cells with Trizol (Life Technologies Inc., Bethesda, MD, Cat. # 15596-026). The mRNA was isolated using the Oligotex mRNA kit (Qiagen Inc., Valencia, CA, Cat. # 70022) according to vendor's protocols. The mRNA was reverse transcribed using Cy3 or Cy5 fluorescent dye to create fluorescently labeled cDNA according to the method described in DeRisi et al., Science, 24:680-686 (1997). The resultant cDNA from TD cells was labeled with Cy3 fluorescent dye and cDNA from normal cells was labeled with Cy5 fluorescent dye (Incyte Genomics, Palo Alto, CA). Microaoay Analysis: To analyze differential gene expression in cells from individuals with TD and normal individuals, Cy3 and Cy5 fluorescent labeled cDNA samples prepared as described above were hybridized to a set of Gene Album microaoays (GEMs) on microscope slides (Incyte Genomics, Palo Alto, CA). Each of six slides contained about 9,800 human cDNA samples plus 200 control samples, resulting in a microaoay of 58,800 partial cDNAs. Therefore, allowing for estimates of redundancy, approximately 30-50% of expressed human genes were represented. The hybridization of Cy3-labeled cDNA prepared from TDl cells and Cy5-labled cDNA from normal cells allowed comparison of the relative RNA content of TD cells versus normal cells for the expressed genes. In addition, Cy3-labeled cDNA prepared from TD2 cells and Cy5-labled cDNA from normal cells were hybridized to the same set of microaoays to examine the variation of gene expression between different TD patients. Results: Data were analyzed using GemTools software (Incyte Genomics, Palo Alto, CA) and expressed as ratios of TD cell to normal cell mRNA. The results indicated that the majority of genes are comparably expressed in TDl and normal cells. As shown in Figure 2 (in the section above and to the left of the diagonal) only 175 genes were more than 2.5-fold underexpressed in TDl cells compared with normal cells, whereas 375 genes were more than 2.5-fold overexpressed in TDl cells compared with normal cells (below and to the right of the diagonal). Genes more highly expressed in the TD cells could include those that are differentially regulated as a consequence of the Tangier mutation, either as a compensatory response or as a contributor to the disease pathology. Among the genes that could contribute to the observed phenotype of TD and are more highly expressed in TD cells include interferon-β (IFN-β), macrophage inflammatory protein-2α, granulocyte chemotactic protein-2, IL- 11 , prostaglandin endoperoxide synthase-2 (COX-2), thrombospondin, and monocyte chemotactic proteins 1, 3, and 4 (Lawn et al., J. Clin. Invest., 104:R25-R31 (1999). No single RNA that was expressed in the normal fibroblasts was found completely absent in either the TDl or TD2 cells. Also, comparison of the differentially expressed genes in TDl and TD2 revealed very little variation between the individual TD patients. For instance, of the most highly down-regulated genes in TD2 cells, 92% were also under-expressed in TD 1 cells compared with normal cells. One of the genes more than 2.5-fold under-expressed in TDl or TD2 versus normal cells was the gene for ABCl protein. The ABCl gene was pursued due to the ascribed functions of some of its homologues and also because the gene was localized to the approximate chromosome region reported as the TD gene region. EXAMPLE 3 This example demonstrates that the ABC 1 gene is localized to the human chromosome 9q31. Previous genetic linkage analysis mapped the TD gene to the 7-cM region of human chromosome 9q31 (Rust et al., Nat. Genet., 20, 96-98 (1998)). In addition, in situ hybridization analyses revealed that the ABCl gene was localized to the broader chromosomal interval 9q22 - 9q31 (Luciani et al., Genomics, 21, 150-159 (1994)). Using PCR methods with the GeneBridge 4 panel of human/hampster radiation hybrids (Research Genetics, Inc., Huntsville, AL), human ABCl was determined to be located between the markers WI- 14706 and WI-4062, cooesponding to the 7-cM region of human chromosome 9q31. DΝA from 93 human/hampster hybrid cell lines was amplified by PCR using human ABCl -specific primers LF: CCTCTCATTACACAAAAACCAGAC (SEQ ID NO: 1 1) and LR: GCTTTCTTTCACTTCTCATCCTG (SEQ ID NO: 12). Each line was scored as positive or negative for the human ABCl amplification product and the mapping of ABCl derived from analysis of this data was accomplished using the Whitehead Institute/MIT Center for Genome Research software, accessed via the internet (http://carbon.wi.mit.edu:8000/cgi- bin/contig/rhmapper.pl). These results were further confirmed by southern blot hybridization to human genomic/yeast artificial chromosome clones (Research Genetics, Inc.) from the equivalent interval. In addition, public database searching (GeneMap '98; National Center for Biotechnology Information) and radiation hybrid mapping eliminated the other significantly underexpressed genes in the microaoay data from the location in the reported genetic interval. These complementary data demonstrate that the ABCl gene is located on human chromosome 9q31 and further indicate that the ABCl gene is associated with Tangier disease. EXAMPLE 4 This example shows the determination of the nucleotide sequence of the wildtype ABCl gene, including the flanking regions and the entire coding region. DNA sequencing was performed using an ABI Prism 310 Genetic Analyzer or by Davis Sequencing (Davis, CA). Both strands were sequenced throughout. The sequence of the open reading frame of the ABCl gene from a normal subject was determined from a full-length cDNA clone obtained from an expression plasmid library constructed from normal fibroblast RNA. To construct the plasmid library, cDNA was synthesized according to the Stratagene kit protocol (Stratagene, La Jolla, CA). Briefly, first strand cDNA was synthesized from mRNA using an oligo-dT primer with an Xhol site and MMLV reverse transcriptase in the presence of 5-methyl dCTP. The second strand was synthesized using RNase H and DNA polymerase I in the presence of unmodified dNTPs. After the cDNA was blunt-ended with pfu DNA polymerase, an EcoRI linker was ligated to the cDNA. The cDNA was then digested with Xhol, creating Xhol ends at the 3' end of the cDNA. The internal Xhol sites were protected from this digestion due to the semi-methylation during the first strand synthesis. The synthesized cDNA was cloned into the Hindlll and Xhol sites of the plasmid pCEP4 (Invitrogen Corp., Carlsbad, CA #VO44-50), an expression vector containing the cytomegalovirus promoter/enhancer. A 585 bp ABCl probe was generated by reverse transcriptase polymerase chain reaction (RT-PCR) using primers based on known ABCl sequence, which were 5'-TCCTTGGGTTCAGGGGATTATC (SEQ ID NO: 13) and 5'-CAATGTTTTTGTGGCTTCGGC (SEQ ID NO: 14). Using this ABCl probe, a clone containing a 10.5 kb insert of human ABCl cDNA was recovered from the library using the CloneCapture selection kit according to the manufacturer's protocol (CLONTECH Laboratories, Inc., Palo Alto, CA). This clone is shown in Figure 3 as pCEPhABCl . The 10.5 kb ABCl cDNA insert sequence is shown in SEQ ID NO: 1. Sequence determination confirmed that pCEPhABCl contains the human ABCl open reading frame of 6783 nucleotides plus 5' and 3' untranslated regions, having a larger open reading frame than the cDNA sequence reported by Langmann et al. in Biochem. Biophys. Res. Comm., 257, 29-33 (1999) (GenBank Accession No. AJO12376). EXAMPLE 5 This example demonstrates the sequence differences between the wildtype ABCl gene and the TDl, TD2, and TD3 gene sequences. cDNA Synthesis of TDl. TD2. and TD3: cDNA was prepared from TDl, TD2, and TD3 cells by reverse transciption polymerase chain reaction (RT-PCR) using the Superscript Choice cDNA system and the Advantage cDNA polymerase mix following the manufacturer's protocol (CLONTECH, Palo Alto, CA; Cat. #8417-1) using two sets of primer pairs designed from the normal human ABCl gene sequence, designated: (1) saclhabcf, 5'- AGTCGAGCTCCAAACATGTCAGCTGTTACTGGAAGTGGCC (SEQ ID NO: 15); habcr3851, 5'- TCTCTGGATTCTGGGTCTATGTCAG (SEQ ID NO: 16) and (2) habcf3585, 5'-GGGAGCCTTTGTGGAACTCTTTC (SEQ ID NO: 17); habcrsall, 5'- ACTGGTCGACCATTGAATTGCATTGCATTGAATAGTATCAG (SEQ ID NO: 18). Amplification of 0.2-0.5 μg polyA-t- RNA with these primers at a final concentration of 0.4μM generated two overlapping templates of approximately 3.5 kb. The templates were gel-purified using the QIAEX II system (QIAGEN, Inc., Valencia. CA; Cat. #20021 ) and adjusted to a concentration of lOOng/μl. Sequencing of TDl . TD2. and TD3 cDNA: Eight μl of each template generated as described above was sequenced in a reaction with individual sequencing primers designed on the basis of wildtype ABCl sequence at a final concentration of 0.5μM. The primers were as follows: IF: 5'- TTTCCTGGTGGACAATGAA (SEQ ID NO: 19), 2F: 5'- AGTGACATGCGACAGGAG (SEQ ID NO: 20); 3F: 5'- GATCTGGAAGGCATGTGG (SEQ ID NO: 21); 4F: 5'- CCAGGCAGCATTGAGCTG (SEQ ID NO: 22); 5F: 5'- GGCCTGGACAACAGCATA (SEQ ID NO: 23); 6F: 5'- GGACAACCTGTTTGAGAGT (SEQ ID NO: 24); 7F: 5'- AAGACGACCACCATGTCA (SEQ ID NO: 25); 8F: 5'- ATATGGGAGCTGCTGCTG (SEQ ID NO: 26); 9F: 5'- GGGCATGAGCTGACCTATGTGCTG (SEQ ID NO: 27); 10F: 5'- AAGAGACTGCTAATTGCC (SEQ ID NO: 28); 1 IF: 5'- AGCGACAAAATCAAGAAG (SEQ ID NO: 29); 12F: 5'- TGGCATGCAATCAGCTCT (SEQ ID NO: 30); 13F: 5'- TCCTCCACCAATCTGCCT(SEQ ID NO: 31); 14F: 5'- TTCTTCCTCATTACTGTT (SEQ ID NO: 32); 15F: 5'- GATGCCATCACAGAGCTG (SEQ ID NO: 33); 16F: 5'- AGTGTCCAGCATCTAAA (SEQ ID NO: 34); 1R: 5'- CAAAGTTCACAAATACTT (SEQ ID NO: 35); 2R: 5'- CTTAGGGCACAATTCCACA (SEQ ID NO: 36); 3R: 5'- TGAAAGTTGATGATTTTC (SEQ ID NO: 37); 4R: 5'- TTTTTCACCATGTCGATGA SEQ ID NO: 38); 5R: 5'- CTCCACTGATGAACTGC (SEQ ID NO: 39); 6R: 5'- GTTTCTTCATTTGTTTGA (SEQ ID NO: 40); 7R: 5'- AGGGCGTGTCTGGGATTG (SEQ ID NO: 41); 8R: 5'- CAGAATCATTTGGATCAG (SEQ ID NO: 42); 9R: 5'- CATCAGAACTGCTCTGAG (SEQ ID NO: 43); 10R: 5'- AGCTGGCTTGTTTTGCTTT SEQ ID NO: 44), 1 1R: 5'- TGGACACGCCCAGCTTCA (SEQ ID NO: 45), 12R: 5'- CCTGCCATGCCACACACA (SEQ ID NO: 46), 13R: 5'- CTCATCACCCGCAGAAAG (SEQ ID NO: 47), 14R: 5'- CACACTCCATGAAGCGAG (SEQ ID NO: 48), 15R: 5'- TCCAGATAATGCGGGAAA (SEQ ID NO: 49), 16R: 5'- TCAGGATTGGCTTCAGGA (SEQ ID NO: 50), UTR1R: 5'- AAGTTTGAGCTGGATTTCTTG (SEQ ID NO: 51). Results: The nucleotide numbering follows the numbering found in Lawn et al. (1999). Patient TD 1 retained the full open reading frame, with 2 substantial differences from the wild- type sequence (SEQ ID NO: 8). One of these is an A to G substitution, resulting in a change from a glutamine to arginine residue at position 537 of the 2201 amino acid sequence, as published by Lawn et al. (1999). The location of this residue is within the NH2-terminal hydrophilic domain, near the first predicted transmembrane domain. Patient TD2 also retained the open reading frame with an arginine to tryptophan substitution at residue 527 (SEQ ID NO: 10). Thus, both TDl and TD2 contain a substitution altering the charge of an amino acid in the same region of the protein. TD3 DNA contains a 14 nucleotide insertion in its ABC l cDNA following nucleotide 5697 in one allele and a 138 bp insertion after nucleotide 5062 in the other allele. Genomic sequencing of the TDl, TD2, and TD3 DNAs confirmed the changes found in the respective cDNAs. The genomic sequence was generated by PCR amplification of a 156 bp region of genomic DNA isolated from fibroblasts that contained the mutations found in the cDNA from TDl and TD2. The genomic sequencing also indicated that patient TDl was homozygous for the glutamine to arginine substitution. Genomic DNA analysis showed that TD2 was a compound heterozygote with one allele containing the detected substitution and the second allele (which failed to produce detectable mRNA) containing an undetermined defect. Neither of the substitution mutations was found in more than 80 alleles of geneomic DNA of non-TD individuals. TD3 insertions were identified by sequence analysis and confirmed by RT- PCR using primers suoounding the insertion points. The 14-bp insertion following nucleotide 5697 causes a frameshift, resulting in the replacement of the wild-type amino acid sequence from a location before the second ATP binding domain, up to the point of a premature protein termination. The 138 bp insertion following nucleotide 5062 in the other allele contains an inframe stop codon. EXAMPLE 6 This example demonstrates that inhibitors of ABCl transport activity also inhibit apo A- I-mediated cholesterol efflux from fibroblast cells. To test whether inhibition of ABCl could affect the process of apolipoprotein-mediated cholesterol efflux, two compounds reported to be ABC 1 inhibitors were tested in assays which monitor apolipoprotein mediated cholesterol efflux. The compounds 4,4-diisothiocyanostilbene- 2,2'-disulfonic acid (DIDS) and sulphobromophthaleine (BSP) were reported to inhibit anion transport activities of ABCl in a dose-dependent fashion (Becq et al., J. Biol. Chem., 272:2695- 2699 (1997); Hamon et al, Blood, 90:2911-2915 (1997)). The apolipoprotein-mediated cholesterol efflux assays were performed as described in Example 1 with the noted changes. Cholesterol-loaded and [ 3 H] cholesterol-labeled normal fibroblasts (n=3) were incubated for 6 hours with or without 5μg/ml apo A-I and either 0, 0.2mM, or 0.4mM DIDS. In addition, cholesterol-loaded and [ 3 H]cholesterol-labeled normal fibroblasts (n=3) were incubated for 6 hours with or without 5μg/ml apo A-I and either 0, 0.2mM, or 0.4mM BSP. [ H] cholesterol efflux was measured by scintillation counting as described in Example 1 and calculated as the percentage of total radiolabeled cholesterol appearing in the medium. The results are shown in Figure 5 as the mean + SD (n=3) of efflux in the presence of apo A-I after subtraction of values for apo A-I-free medium. Figure 5 shows that both DIDS and BSP inhibit the 6-hour efflux of tritiated cholesterol mediated by apolipoprotein A-I. In addition, similar inhibition was observed with the efflux of tritiated phosphatidyl choline using DIDS and BSP (data not shown). The results of these tests mimic the efflux defect in fibroblasts derived from patients with TD, described in Example 1. EXAMPLE 7 This example demonstrates that antisense inhibition of ABCl mRNA expression inhibits apo A-I-mediated cholesterol efflux from fibroblast cells. Normal skin fibroblasts were labeled with [ 3 H] cholesterol as described in Example 1. The cells were then loaded with oligonucleotide by scraping in the presence of either 30μM control Morpholino oligonucleotide (5'-CCTCTTACCTCAGTTACAATTTATA-3' cooesponding to the antisense complement of a β-globin thalassemic mRNA; SEQ ID NO: 52) or 30μM ABCl antisense Morpholino oligonucleotide (5'- CATGTTGTTCATAGGGTGGGTAGCTC-3'; SEQ ID NO: 53) and reseeding on new dishes. Control cells were mock-loaded after [ 3 H]cholesterol-labeling by scraping and reseeding in the absence of oligonucleotide. Apo A-I-mediated efflux was measured after 12 hours by scintillation counting as the percentage of total radiolabeled cholesterol appearing in the medium. The results are shown in Figure 6 as the mean + SEM of three separate experiments, normalized to the value for apo A-I-specific efflux in the absence of oligonucleotide in each experiment. As shown in Figure 6, antisense oligonucleotides directed against ABCl mRNA caused a 50% reduction in cholesterol efflux from normal fibroblasts compared with control antisense oligonucleotide (β-globin antisense oligonucleotide). EXAMPLE 8 This example demonstrates that over expression of the human ABC 1 gene results in an increase in apo A-I-mediated cholesterol efflux from monocyte cells. Stable Transfection of RAW 264.7 Cells: Mouse monocytic RAW 264.7 cells were stably transfected with the pCEPhABC 1 expression plasmid for human ABC 1. Construction of the pCEPhABCl plasmid containing the open reading frame of human ABCl is described in Example 4. Approximately 1 x 10 6 RAW 264.7 cells were transfected for 5 hours with 2 μg of pCEPhABCl DNA and 12μl Geneporter transfection reagent (Gene Therapy Systems, Inc., San Diego, CA; Cat. #T201007) in 0.8ml serum-free DMEM. Two days later, cells were split at ratios ranging from 1 :2 - 1 :50 and selection applied by adding 150 μg/ml hygromycin to the culture medium. After two weeks, hygromycin-resistant colonies were picked and expanded. Apo A-I-mediated Cholesterol Efflux Assay: Parental RAW 264.7 cells and three clonal lines (L3, L5, and L6) stably expressing human ABCl were grown to confluence. The cells were cholesterol-loaded and labeled by incubation for 24 hours with 0.5 μCi/ml [ 3 H] cholesterol and 50μg/ml acetylated LDL as described in Example 1. After equilibrium of cholesterol pools by an overnight incubation in DMEM/BSA, cells were washed and the efflux medium was added as described in Example 1. Apo A-I-mediated cholesterol efflux was measured as previously described by scintillation counting of the tritiated cholesterol in the cell medium, expressed as a percentage of the total counts recovered from the cells and medium. The results are presented as the mean + SEM of three separate experiments normalized to the value for apo A-I-specific efflux from parental RAW 264.7 cells within each experiment. Figure 7 shows the apo A-I- mediated cholesterol efflux from parental RAW 264.7 cells and L3, L5, and L6 transfected cell lines. As can be seen, transfection with the ABCl expression vector results in a 4-fold (L6) to 8- fold (L3 and L5) increase in apo A-I-mediated cholesterol efflux. These results indicate that overexpression of the ABCl gene can substantially increase the amount of cholesterol efflux from macrophage cells. EXAMPLE 9 This example demonstrates that ABCl mRNA expression is regulated by cellular conditions related to cholesterol efflux in normal skin fibroblasts, but not in TD fibroblasts. To determine whether ABC 1 plays a rate-limiting role in cellular sterol efflux, the synthesis of ABCl was measured under various cellular conditions related to cholesterol efflux processes. Specifically, normal fibroblasts and TD fibroblasts were individually exposed to conditions of excess cAMP, cholesterol, or Apo A-I. Cell cultures of normal skin fibroblasts and TDl and TD2 fibroblasts were prepared as described in Example 1. The level of ABCl mRNA was measured by RT-PCR. Cell cultures: Immortalized cell cultures of normal skin fibroblasts and TDl and TD2 fibroblasts were prepared as described in Example 1. Cells were grown to subconfluence in DMEM 10% FBS before replacement with DMEM BSA and the indicated additive for 24 or 48 hours. RNA was prepared as described in Example 2. RT-PCR: Quantitative PCR was carried out using the GeneAmp 5700 Sequence Detection System (Perkin-Elmer Applied Biosystems, Foster City, CA). Briefly, 500ng of DNase-treated mRNA was reverse transcribed using random hexamer primers at 2.5μM. Approximately 5% of this reaction was amplified by PCR using the SYBR green core kit (PE Applied Biosystems, Foster City, CA; Cat. #4304886) and human ABCl primers LF: 5'- CCTCTCATTACACAAAAACCAGAC (SEQ ID NO: 11) and LR: 5'- GCTTTCTTTCACTTCTCATCCTG (SEQ ID NO: 12) to yield an 82 bp fragment cooesponding to nucleotides 7018-7099 of human ABCl . PCR cycle conditions were as follows: 10 minutes 95°C; followed by 40 cycles of 95°C, 15 seconds; and 60°C, 60 seconds. The mRNA in each sample was quantitated by detecting the increase in fluorescence caused by SYBR green binding to the double-stranded amplification product generated during each PCR cycle. All samples were run in triplicate and normalized against β-actin mRNA, amplified in parallel reactions with primers actinF: 5'-TCACCCACACTGTGCCATCTACGA (SEQ ID NO: 54) and actinB: 5'- CAGCGGAACCGCTCATTGCCAATGG (SEQ ID NO: 55). Standard curves were run for both ABCl and β-actin on the same PCR plate. 8-Br-cAMP Assay: Normal, TDl, and TD2 fibroblast cells were grown to subconfluence in DMEM/10% FBS and then treated with 1 mM 8-Br-cAMP in DMEM/BSA for 24 hours. Cholesterol Assay: Normal, TD 1 , and TD2 fibroblast cells were grown to subconfluence in DMEM/10% FBS and then treated with 30μg/ml free cholesterol in DMEM/BSA for 48 hours followed by 18-24 hours of equilibrium in DMEM/BSA. Apo A-I Assay: Normal, TDl, and TD2 fibroblast cells were grown to subconfluence in DMEM/10% FBS and then treated with 30μg/ml free cholesterol in DMEM/BSA for 48 hours followed by 18-24 hours of equilibrium in DMEM/BSA plus lOμg/ml apo A-I. Results: Figure 8 shows that in normal fibroblasts ABC 1 mRNA is increased approximately 10-fold by exposure to 8-Br-cAMP and increased approximately 17-fold by exposure to cholesterol in serum-free medium. Subsequent exposure of cholesterol-loaded cells to Apo A-I results in a marked decrease in ABCl mRNA expression. Although the mechanism has not been demonstrated, previous studies have shown that cholesterol efflux is promoted in the presence of such compounds as cAMP and cholesterol (Hokland et al., J. Biol. Chem., 268:25343-25349 (1993)). The present results indicate that in normal fibroblasts, ABCl mRNA is induced by these known effectors of the cholesterol efflux pathway and repressed by exposure to an apolipoprotein cholesterol acceptor, demonstrating that the expression of ABCl is regulated by cellular conditions related to apolipoprotein-mediated cholesterol efflux. In contrast, fibroblast cells from TD patients are not regulated by effectors of cholesterol efflux. First, the cAMP-inducible level of ABCl mRNA in both TDl and TD2 cells is only approximately 40% of that in normal cells. Further, exposure of cholesterol-loaded cells to Apo A-I either did not alter ABCl expression (TDl cells) or slightly increased ABCl expression (TD2 cells). These results reflect the defect in Apo-A-I-mediated cholesterol efflux described for TD cells. Interestingly, growth of cells in serum-containing medium suppressed ABCl message to near the limit of detection (data not shown). This may reflect the fact that functioning of the lipid efflux pathway requires cell quiescence or other cellular states of reduced cholesterol need. In conclusion, conditions that are associated with increased efflux of cellular cholesterol (i.e., cholesterol loading, cAMP treatment, serum deprivation) also result in increased expression of ABCl mRNA in normal fibroblast cells. Conversely, exposure of cholesterol-loaded normal fibroblast cells to apo A-I reduces ABCl expression. EXAMPLE 10 This example demonstrates that ligands for the LXR nuclear receptor, such as 20- hydroxycholesterol, and ligands for the RXR receptor, such as 9-cis retinoic acid, can increase ABCl gene expression in mouse RAW 264.7 cells. LXR nuclear receptors are transcription factors that form obligate heterodimers with the nuclear receptor RXR, and are activated to enhance transcription of their target genes by binding a class of oxysterols including 22-hydroxycholesterol and 20-hydroxycholesterol (Janowski et al., Nature, 383:728-731 (1996)). As such, they are candidates for the mediation of cholesterol- induced gene transcription. Further, in light of studies which showed that ABCl mRNA and protein increase in fibroblasts and macrophages in response to cholesterol loading, and other studies which showed that LXR and RXR expression increase in cholesterol-loaded macrophage cells by exposure to oxidized LDL, the LXR and RXR nuclear receptors are highly plausible candidates for transcriptional activators of the ABCl gene. To determine whether LXR and RXR receptors play a role in ABCl gene expression, the level of ABCl mRNA was measured in response to 20-hydroxycholesterol and 9-cis retinoic acid. Mouse RAW 264.7 cells were grown to subconfluence in DMEM/10% FBS and then treated for 24 hours in serum-free DMEM/BSA with either 9-cis retinoic acid (lOμM), 20- hydroxycholesterol (lOμM), or both ligands together (20μM total). Control cells received ethanol vehicle only (0.1% v/v). RNA was extracted, treated with DNase, and ABCl mRNA measured by RT-PCR using PE Biosystems SYBR Green Technology as described in Example 9. Figure 9 shows that treatment with either 20-hydroxycholesterol or 9-c/s retinoic acid results in an increase in ABCl mRNA expression. In addition, Figure 9 shows that treatment with both ligands together results in a markedly synergistic effect, with an approximate 6-fold increase over the ABCl expression observed with either ligand alone. These results demonstrate that ligands for the nuclear receptors LXR and RXR can increase the expression of the ABCl gene. EXAMPLE 1 1 This example demonstrates that enhanced expression of ABCl protein in the plasma membrane is associated with lipid efflux. Cell-surface labeling and immunoprecipitation was used to determine whether increased expression of ABC 1 protein in the plasma membrane is cooelated with an increase in cholesterol efflux (Figure 10). The relative amount of ABCl on the cell surface was determined by cross- linking surface proteins on intact cells with the membrane-impermeable agent sulfo-NHS-biotin, followed by the steps of membrane solubilization, immunoprecipitation with ABCl antibody, SDS-PAGE, and detection with streptavidin. Cell Culture: Normal and TD 1 fibroblast cells were immortalized as described in Example 1. Both normal and TDl cells were cultured under control conditions and conditions known to increase apolipoprotein-mediated cholesterol efflux (Oram, et al., J. Lip. Res., 40: 1769-1781 (1999)). Control cells were grown to confluence in DMEM/10% FBS and then incubated in DMEM/BSA for 18 hours with no additives (control). cAMP-treated cells were grown to confluence in DMEM/10% FBS and then incubated in DMEM/BSA for 18 hours with ImM 8-Br-cAMP(cAMP). Cholesterol-loaded cells were grown to confluence in DMEM/10% FBS and then incubated in DMEM/BSA for 48 hours with 30μg/ml cholesterol plus 18 hours with no additives (cholesterol). Cholesterol-loaded cells treated with cAMP were grown to confluence in DMEM/10% FBS and then incubated in DMEM/BSA for 48 hours with 30μg/ml cholesterol plus 18 hours with ImM 8-Br-cAMP (cholesterol + cAMP). Cell-Surface Labeling: For selective labeling of plasma membrane ABCl, the cells were incubated for 30 minutes at 0°C with PBS containing 1 mg/ml sulfo-NHS-biotin (Pierce, Rockford, IL; Cat. #21217) to biotinylate cell-surface proteins (see Walker et al., Biochemistry, 50: 14009-14014 (1993)). Immunoprecipitation: Rabbit antiserum for ABC 1 was raised against a synthetic peptide cooesponding to the deduced peptide KNQTVVDAVLTSFLQDEKVKES located at the C- teoninus of human ABCl . Immunoprecipitation was performed by solubilizing the cells in PBS containing 1% Triton X-100 (Sigma, St. Louis, MO) and protease inhibitors leupeptin (ImM), pepstatin (ImM), and aprotinin ( ImM). The cell extract was incubated overnight at 4°C with anti-ABCl antiserum at 1 :200 dilution followed by an additional 1 hour incubation with 5μl proteinA-coated magnetic beads (Dynal, Lake Success, NY; Cat. #1001.01). The antibody- antigen complex was sedimented with a magnet, the beads were washed twice with 1 % Triton- X/PBS, and the proteins were eluted with 1% acetic acid. Detection of ABCl Protein: The eluted biotinylated proteins were subjected to SDS- PAGE (6% gel; 150V, 5 hours) and transfeoed to nitrocellulose membrane (200mA, 18 hours). The nitrocellulose was probed with streptavidin-horse radish peroxidase (Amersham Pharmacia, Piscataway, NJ; Cat. #RPN 1231) diluted 300-fold and detected by enhanced chemiluminescence labeling (ECL) according to vendor's protocol (Amersham Pharmacia, Piscataway, NJ). To test for possible biotinylation of intracellular proteins, the post- immunoprecipitation supernatant was treated with a mouse monoclonal antibody to the intracellular protein β-COP and immunoprecipitated biotinylated β-COP was assayed by streptavidin blotting. None was detected. Results: As shown in Figure 10, the 240 kDa ABCl protein appears as a doublet. The ABCl protein is partially localized to the plasma membrane in both normal (10A) and TDl (10B) fibroblast cells. Similar results were seen with a second normal fibroblast cell line and with TD2 fibroblasts (data not shown). Cell-surface expression of ABCl was increased slightly when cells grown in serum (normal and TDl cells) were treated with 8-Br-cAMP. Serum deprivation and cholesterol-loading of both normal and TDl cells markedly increased cell- surface expression of ABCl, which was further enhanced by cAMP treatment. These results indicate that expression of ABCl at the cell surface is regulated by conditions that enhance apolipoprotein-mediated lipid efflux, consistent with the idea that its localization to the plasma membrane plays a key role in its lipid transport function. The mutations in TDl and TD2 cells do not appear to severely impair expression or processing of ABCl, implying that secondary effects on lipid transport or interactions with accessory proteins depend on its NH2-terminal domain, where the mutations occur. EXAMPLE 12 This example shows that agents that inhibit the degradation of 3'5' cyclic AMP, such as phosphodiesterase inhibitors, increase apolipoprotein A-I-mediated efflux from macrophage cells. As shown in Figure 10, cAMP increases the activity of ABCl . The present studies were performed to determine the cholesterol efflux from macrophage cells in the presence of elevated cAMP. Elevated levels of cAMP can be attained in the presence of agents that either stimulate cAMP synthesis or inhibit the degradation of cAMP. For example, rolipram is a compound that regulates cAMP levels by inhibiting phosphodiesterases, a group of enzymes that degrade cAMP. The effect of elevated cAMP on cholesterol efflux was determined using the apolipoprotein-mediated cholesterol efflux assays described in Example 1. Briefly, RAW 264.7 cells suspended at a density of 1.25 x 10 5 cells/ml were grown in DMEM/10% FBS supplemented with pyruvate. After 24 hours, the medium was removed and replaced with DMEM/BSA plus radiolabeled cholesterol (1 μCi/ml 3 [H]-cholesterol) and 50 μg/ml of acetylated LDL for 24 hours. The cells were then maintained for 24 hours in equilibrium medium consisting of DMEM/BSA plus either apo A-I alone (20 μg/ml), apo A-I and 8-bromo 3', 5' cAMP (ImM) or apo A-I and rolipram (50 μM). After 12-24 hours, [ 3 H]cholesterol efflux was measured by scintillation counting as described in Example 1 and calculated as the percentage of total radiolabeled cholesterol appearing in the medium. The results indicated that cholesterol-loaded control cells that received no apo A-I showed a 3% cholesterol efflux, while cells that received apo A-I only showed a 5% efflux. Cholesterol-loaded cells that received apo A-I and cAMP showed a 32% cholesterol efflux, demonstrating that elevated cAMP promotes cholesterol efflux. Similarly, cells that received apo A-I and a phosphodiesterase inhibitor (rolipram) showed a 17% cholesterol efflux. EXAMPLE 13 This example shows that agents that are ligands for nuclear receptors, such as LXR, RXR, and PRAR nuclear receptors, increase apolipoprotein A-I-mediated efflux from macrophage cells. To determine whether ligands for nuclear receptors affect the process of apolipoprotein- mediated cholesterol efflux, various ligands were tested using the apo A-I-mediated cholesterol efflux assay described in Example 12. The nuclear receptor superfamily includes several members, such as the liver receptor LXR, the retinoid receptor RXR, and the peroxisome proliferator-activated receptor PPAR, which have been implicated in lipid metabolism (Russell. D.W., Cell, 97:539-542 (1999); Spiegelman, B.W., Cell, 93: 153-155 (1998); Janowski et al., Nature, 383:728-731 (1996)). Further, ligands for some of these receptors have been observed to increase plasma HDL and gene expression profiling (microaoay) data have shown that hormone receptors respond to cholesterol loading via oxidized LDL. Using the above-described assay, 9 cz ' s-retinoic acid (RXR ligand), oxysterol (LXR ligand), and fenfibrate (PPAR ligand) was tested to determine the effect on cholesterol efflux. Cholesterol-loaded control cells that received no apo A-I showed a 3% cholesterol efflux, while cells that receive apo A-I only showed a 5% efflux. In contrast, cholesterol-loaded cells that receive apo A-I and 9 c/s-retinoic acid (30 ng/ml) showed a 16% cholesterol efflux. Cells that receive apo A-I and oxysterol (5 μg/ml) showed a 14% cholesterol efflux. Cells that receive apo A-I and fenfibrate (3 μg/ml) showed a 10% cholesterol efflux. These results indicate that hormone receptors may be modulated to increase the rate of apolipoprotein-mediated cholesterol efflux from macrophages. Further, when the efflux assay was performed using various concentrations of 9-cis-RA (0.3ng/ml, 3.0ng/ml, or 30ng/ml), the results showed that 9-cis-RA mediated cholesterol efflux from macrophage cells in a dose-dependent manner. Specifically, control cells (apo A-I only) showed 1890 c.p.m., 0.3 ng/ml 9-c/-f-RA showed 1522 c.p.m., 3.0 ng/ml 9-cw-RA showed 3568 c.p.m., and 30 ng/ml 9-cw-RA showed 8597 c.p.m. In addition, using a similar assay where RAW 264.7 cells were cholesterol-loaded for 48 hours, other nuclear receptor activators, such as 22-hydroxycholesterol (LXR ligand) and benzfibrate, were shown to increase cholesterol efflux (data not shown). EXAMPLE 14 This example shows that eicosanoids, such as prostaglandin El and prostacyclin PG12, increase apolipoprotein A-I-mediated efflux from macrophage cells. Eicosanoids, such as prostaglandins and prostacyclins, have been shown to be effective in the treatment of hypercholesterolemia. To determine whether eicosanoids affect the process of apolipoprotein-mediated cholesterol efflux, PGE1 and PG12 were tested using the apo A-I- mediated cholesterol efflux assay described in Example 12. This assay showed that cholesterol- loaded control cells that receive no apo A-I have a 3% cholesterol efflux, while cells that receive apo A-I only have a 5% efflux. Cholesterol-loaded cells that receive apo A-I and PG12 (25 nm) showed a 10% cholesterol efflux. Cells that receive apo A-I and PGE1 (25 nM) showed a 15% cholesterol efflux. These results demonstrate that eicoasnoids can increase the rate of apolipoprotein-mediated cholesterol efflux from macrophages. EXAMPLE 15 This example demonstrates that a reporter gene under the control of an ABC 1 promoter can be used to test compounds for the ability to regulate ABC 1 gene expression. The pGL3 luciferase reporter vector system (Promega, Madison, WI) was used to create a recombinant plasmid to measure reporter gene expression under control of the ABCl promoter. Construction of Reporter Plasmids: Plasmid pGL3-Basic (Promega, Madison, WI; Cat. #E1751) was used as a control plasmid containing the promoterless luciferase gene. The reporter construct containing the ABCl promoter and luciferase gene was made by cloning a genomic fragment from the 5' flanking region of the ABCl gene (hAPRl 5' promoter, cooesponding to nucleotides 1080-1643 of SEQ ID NO: 3) into the Sacl site of the GL3-Basic plasmid to generate plasmid GL-6a. Next, plasmid GL-6a was digested with Spel and Acc65I. A BsiWI-Spel fragment excised from a lambda subclone, representing the ABCl genomic sequence cooesponding to nucleotides 1-1534 of SEQ ID NO: 3 was ligated into the remaining vector/ ABCl promoter fragment produced by this digestion. The resultant plasmid, pAPRl, encodes the luciferase reporter gene under transcriptional control of 1.75 kb of the human ABCl promoter sequence. Transfection of Reporter Constructs: The above-described control or pAPRl plasmid was transfected into confluent cultures of RAW 264.7 cells maintained in DMEM containing 10% fetal bovine serum. Each well of a 12 well dish was transfected for 5 hours with either pGL3-Basic, pGL3-Promoter or pAPRl DNA (l μg), luciferase plasmid DNA (1 μg), and 12 μl of Geneporter reagent (Gene Therapy Systems, San Diego, CA; Cat. #T201007). In addition, 0.1 μg of pCMVβ plasmid DNA (Clontech, Palo Alto, CA, Cat. #6177-1 ) was added as a control for transfection efficiency. After 5 hours, the culture medium was replaced with serum-free DMEM/BSA in the presence of or absence of acetylated LDL (100 μg/ml) and incubated for 24 hours. For added convenience in high throughput screening, cultured cells can be stably transfected with reporter plasmids using the following procedure. First, 5xl0 6 RAW 264.7 cells are transfected for 5 hours in a 60mm dish with 9μg of the pAPRl plasmid and pCMVscript (Stratagene, LaJolla, CA) in 10 ml of serum-free DMEM with 50 μl Geneporter transfection reagent (Gene Therapy Systems, San Diego, CA). Subsequently, the transfection medium is replaced with complete medium and the cells incubated overnight at 7°C. Subsequently, the cells are transfeoed to separate dishes at dilutions ranging from 1 :5 to 1 : 1000 and incubated in selection medium containing 800 μg/ml G418 (Life Technologies, Bethesda, MD) for 20 days. Visible colonies are picked, expanded, and assayed for luciferase activity as described below. Using this method, five clonal cell lines positive for luciferase activity were identified for use in high throughput assays. Luciferase Assay: Following transfection, the cells in each well were lysed in 70 μl of IX cell lysis reagent (Promega, Madison, WI, Cat. #E3971 ), subjected to one freeze-thaw cycle, and the lysate cleared by centrifugation for 5 minutes at at 12,000g. After centrifugation, 100 μl of luciferase assay reagent (Promega, Madison, WI; Cat. #E1501) was added to 10 μl of lysate. The luciferase activity of each lysate was measured as light units using a luminometer. Additionally, the β-galactosidase activity of each lysate was measured using the chemiluminescent assay reagents supplied in the Galacto-light kit according to the manufacturer's instructions (Tropix Inc., Bedford, MA: Cat. #BL100G). The normalized luciferase activity for each lysate was determined by dividing the luciferase activity value by the determined β-galactosidase value and reported as relative light units. Results: The luciferase activity detected in cells transfected with pAPRl was 3.3-fold higher than the activity detected in control cells transfected with pGL3-Basic plasmid containing luciferase cDNA only. These results indicated that the transcriptional regulatory regions of ABCl were in place. When the pAPRl transfected cells were incubated with 100 μg/ml acetyl LDL for 24 hours, the luciferase activity was 3.25-fold higher than in cells that had not been treated with acetyl LDL. These results suggest that the genomic ABCl sequence contains a "cholesterol responsive" element found in the 5'flanking region which mediates the cholesterol loading response of the native ABCl gene. This reporter system can also be used to test other compounds to determine whether the compound modulates ABCl expression. EXAMPLE 16 This example demonstrates an additional assay that can be used to test compounds for the ability to regulate ABC 1 gene expression using a reporter gene under the control of an endogenous ABCl promoter. This assay involves constructing a recombination vector that contains a promoterless reporter gene and a selection marker gene. The vector is linearized and transfected into cells such that the reporter gene is integrated into the cellular genome downstream of the endogenous ABCl promoter. Using this assay, expression of the reporter gene is driven by the endogenous ABC 1 promoter in response to a test compound. Construction of Reporter Plasmids: The recombination vector containing a promoterless reporter gene can be made starting with a 7 kb EcoRI genomic fragment of ABCl that contains exon 0 (which includes the ABCl start sites) and part of intronl . Using site-directed mutagenesis, a Sail restriction site can be generated in the exon 0 sequence downstream of the two known start sites. The recombination vector is generated by inserting a DNA fragment containing a promoterless reporter gene, such as luciferase, and a promoterless selective marker, such as puromycin resistance, into the Sail site. An internal ribosome entry signal should be inserted between the reporter gene and marker gene so that the genes will be transcribed in the cooect orientation. The recombination vector contains two Eco47III sites, one of which must be eliminated, using, for example, site-directed mutagenesis. The remaining Eco47III site, located upsteam of of exon 0, is used to linearize the vector. Transfection of Reporter Constructs: The linearized recombination vector containing the reporter gene and marker gene is introduced into cultured cells, including human cells, by any of the various transfection methods known in the art. For example, the linearized vector can be transfected using the methods described in Example 15. The linearized recombination vector contains ABCl sequences which allow the vector to integrate into the cellular genome at the site of the endogenous ABCl gene. The addition of an appropriate antibiotic to the culture medium allows the selection of only those cells in which the reporter gene and marker gene have integrated downstream of the endogenous ABCl promoter in the proper orientation. For instance, if the vector contains a puromycin resistance gene inserted downstream of the reporter gene, the transfected cells should be grown in the presence of puromycin. Only those cells that have a properly integrated puromycin resistance gene, and can thereby encode a functional protein, will survive in the presence of puromycin. Thus, the transfected cells should be grown under conditions that induce ABCl promoter activity in the presence of the appropriate antibiotic. Surviving cells can be clonally cultured and the DNA sequenced using PCR or southern blot analysis to test for proper integration of genomic sequences. The resultant cells containing a reporter gene under the control of the endogenous ABCl promoter can be used to determine whether a test compound modulates the expression of ABC 1. The ABCl modulating activity of a compound is determined by assaying the level of reporter gene expression found in the cells exposed to the test compound. For example, cells having an integrated luciferase gene can be used to determine the ABCl modulating activity of a test compound by measuring the amount of luciferase activity found in cells exposed to the compound. EXAMPLE 17 This example demonstrates that ligands for nuclear receptors up-regulate the expression of a reporter gene under the control of the ABCl promoter. To determine whether ligands for the LXRα, LXRβ and RXRα nuclear receptors could regulate ABCl gene expression, the pAPRl plasmid containing the luciferase reporter gene under control of the ABCl promoter was transfected into RAW 264.7 cells treated with at least one ligand for the nuclear receptors (Figure 12). Construction and Transfection of Reporter Constructs: Reporter construct pAPRl and control reporter construct pGL3-Basic were obtained as described in Example 15. RAW 264.7 cells were maintained in culture and transfected with either pGL3-Basic (lμg) or pAPRl (lμg) as described in Example 15. The transfected RAW 264.7 cells were treated with either ethanol (EtOH) (0.1% v/v), 20(S)-hydroxycholesterol (20(S) OH-chol) (10μM), 9-c/s retinoic acid (9-c/s RA) (lOμM) or both 20(S) OH-chol and 9-cis RA (20μM total) for24 hours. The luciferase activity was measured and reported as relative light units as described in Example 15. Results: The results of this study are shown in Figure 12. Control cells transfected with pGL3-Basic showed no luciferase activity (data not shown). Cells transfected with pAPRl produced a 19-fold increase in luciferase reporter activity in the presence of 20 OH-chol, a 16- fold increase in luciferase activity in the presence of 9-cis RA, and a 280-fold increase in luciferase activity in the presence of both ligands compared with EtOH control. These results indicate that both the sterol and retinoid elicit a strong transcription response from the ABCl 5' flanking sequence in pAPRl . Further, there is an apparent synergistic effect of the two classes of compounds, as can be seen by the dramatic increase in luciferase activity found in cells treated with both ligands. It is known that LXRα and RXRα receptors form active heterodimers. Thus, the ligand-induced activation of both nuclear receptors simultaneously could produce the observed synergistic increase in transcription. These data demonstrate that hydroxy sterols, such as 20(S) hydroxycholesterol, and retinoids, such as 9-cis retinoic acid, activate the ABCl promoter, indicating that these and related compounds can be useful in the development of therapeutic compounds to increase ABCl expression in macrophage cells to rid peripheral sites of excess cholesterol. Additionally, the present ABCl promoter/reporter gene screening assay can be used to screen other compounds that increase ABCl expression to identify further therapeutic compounds. EXAMPLE 18 This example demonstrates the further characterization of the ABC l promoter region, including the identification of an LXR response element. To determine which portion of the 5' flanking region of ABCl retains transcriptional activity in response to nuclear ligands, various plasmids containing a different portion of the 5' flanking region and a luciferase reporter gene were transfected into RAW 264.7 cells treated with at least one ligand for the nuclear receptors. Using this system, an sterol response element cooesponding to nucleotides 1480-1510 of SEQ ID NO: 3 was identified. The sterol response element contains a direct repeat-4 element TGACCGatagTAACCT. Confirmation of the sterol response element was obtained using site-directed mutagenesis and band-shift assay techniques. Construction of Reporter Constructs: Reporter construct pAPRl and control reporter construct pGL3-Basic were obtained as described in Example 15. Reporter constructs containing either nucleotides 1-1532, 1080-1643, 1 181-1643, 1292-1643, or 1394-1643 of SEQ ID NO: 3 were also constructed. A reporter construct containing nucleotides 1080-1643 of SEQ ID NO: 3 (GL-6a) was constructed as described in Example 15. A reporter construct containing nucleotides 1-1532 of SEQ ID NO: 3 was constructed by digestion of pAPRl with Spe l and Nhe I, and re-ligation of the gel-purified vector fragment. A reporter construct containing nucleotides 1181 to 1643 was constructed by firstly digesting GL-6a with Sty I. blunting the cohesive ends with Klenow enzyme, digesting the resultant vector with Sac I. and isolating the 462 base pair blunt-Sac I cohesive end fragment. This was cloned into a vector obtained by digestion of GL-6a with Acc65 I. blunting of the cohesive ends with Klenow enzyme, digestion with Sac I and gel isolation of the vector fragment. A reporter construct containing nucleotides 1292-1643 was constructed by consecutive digestion of GL-6a with Acc65 I. blunting the ends with Klenow enzyme, digestion with Sac II. blunting the ends with T4 polymerase, and re-ligation of the gel- isolated vector fragment. A reporter construct containing nucleotides 1394-1643 was constructed by digestion of GL-6a with Acc65 I, blunting the ends with Klenow enzyme, subsequent digestion with Apa I, end-blunting with T4 polymerase and re-ligation of the gel-isolated vector fragment. Transfection of Reporter Constructs: The RAW 264.7 cells were maintained in culture and transfected with either pGL3-Basic (lμg), pAPRl (lμg), or one of the other reporter constructs according to the method described in Example 15. The transfected RAW 264.7 cells were treated with either ethanol (EtOH) (0.1% v/v), 20(S)-hydroxycholesterol (20(S) OH-chol) (lOμM), 22(R)-hydroxycholesterol (22(R) OH-chol) (lOμM), 9-c/s retinoic acid (9-c/s RA) (lOμM), or both 20(S) OH-chol and 9-c/s RA (20μM total) for 24 hours. The luciferase activity was measured and reported as relative light units as described in Example 15. Site-Directed Mutagenesis: The sterol response element cooesponding to nucleotides 1480-1510 of SEQ ID NO: 3 was mutated in the 1080-1643 sequence described above using site- directed mutagenesis. Specifically, the response element containing a direct repeat-4 element TGACCGatagTAACCT was mutated to CTGCACatagTAACCT using the GeneEditor system (Promega, Madison, WI) according to the manufacturer's protocol. Gel-Shift Assays: Nuclear extract was prepared from RAW 264.7 cells by the method of Ohlsson et al., Cell, 45:35-44 (1986). 32 P-labeled oligonucleotides (5 ng) cooesponding to the LXR response element (TCGAGTGACCGATAGTAACCTCTCGA; SEQ ID NO: 56) and its mutated counterpart (TCGAGCTGCACATAGTAACCTCTCGA; SEQ ID NO: 57) were individually incubated with 5 μg of nuclear protein for 30 minutes at room temperature in 20mM HEPES, pH 7.9, 60mM KCL, ImM MgCl 2 , ImM DTT, 66.6μg/ml poly(dldC), and 10% glycerol in the presence or absence of 1 μg antiserum to LXRα and LXRβ (Santa Cruz Biotechnology, Cat. No. SC-1591, Santa Cruz, CA) or antiserum to RXR (Santa Cruz Biotechnology, Cat. No. SC-774, Santa Cruz, CA). The protein-DNA complexes were applied to a 4% non-denaturing polyacrylamide gel for 1.5 hours at 150V in 0.5X TBE buffer. The protein- DNA complexes were detected by autoradiography of the dried gel. Results: Transfection with the individual reporter constructs containing the 5' flanking region cooesponding to nucleotides 1-1643 (i.e., pAPRl), 1-1532, 1080-1643, 1181-1643, 1292- 1643, or 1394-1643 of SEQ ID NO: 3 each produced the same results. All of the individual constructs produced a 3 to 4- fold increase in luciferase reporter activity in the presence of 20 (S) OH-chol or 22 (R) OH-chol compared with EtOH control. Also, all of the individual constructs produced an 8 to 10-fold increase luciferase reporter activity in the presence of 9-c/s RA. In addition, transfection with any of the constructs produced a 25 to 50-fold increase in luciferase activity in the presence of oxysterol ligand (either (20 (S) OH-chol or 22(R) OH-chol)) and retinoid ligand (9-c/s RA) together compared with EtOH control, indicating a synergistic interaction. Each of the described constructs demonstrated comparable levels of luciferase activity in response to the ligands tested, indicating that even the shorter 5' flanking sequences contained transcriptional regulatory sequences for sterols and retinoids. Specifically, these results demonstrated that the transcriptional regulatory sequences for sterols and retinoids are located in the 5' flanking region cooesponding to nucleotides 1394-1532 of SEQ ID NO: 3. These results were confirmed by luciferase assays using a reporter construct containing the wild-type sequence cooesponding to nucleotides 1080-1643 of SEQ ID NO: 3 and a reporter construct containing a mutated sequence cooesponding to nucleotides 1080-1643 of SEQ ID NO: 3, wherein the sterol response element found at nucleotides 1480-1500 was mutated as described above. Transfection with the wild-type sequence produced a transcritional response, as measured by an increase in luciferase activity, in the presence of either 20 (S) OH-chol or 9-c/s RA alone and produced a synergistic response in the presence of both ligands together. In contrast, transfection with the mutated sequence did not produce a transcriptional response in the presence of 20 (S) OH-chol or 22 (R) OH-chol. Transfection of the mutated sequence preserved a reduced response to 9-c/s RA, producing a 4 to 5-fold increase in transcriptional activity, rather than the 8 to 10-fold increase observed with the wild-type sequence. Transfection of the mutated sequence also abolished the synergistic transcriptional response seen in the presence of 20 (S) OH-chol and 9-c/s RA together. These results were further confirmed by gel-shift assays using the sterol consensus sequence (nucleotides 1480-1510) and its mutated counterpart. The gel-shift assays showed that while nuclear binding proteins isolated from RAW 264.7 cells bound to the sterol consensus sequence, nuclear proteins did not bind to the mutated sequence. Furthermore, incubation of nuclear proteins with the wild-type sterol consensus sequence in the presence of LXR antiserum resulted in the formation of supershifted complexes (i.e. antibody-protein-DNA complexes), identifying the sequence as a sterol response element that binds nuclear receptor LXR. In contrast, incubation of nuclear proteins with the wild-type sterol response element in the presence of RXR antiserum did not result in the formation of supershifted complexes, indicating that RXR does not bind to this sequence. These results show that the mutation which destroys nuclear protein binding to the consensus sequence also abolishes the transcriptional response to LXR ligands and diminishes the response to RXR ligands. Furthermore, the nuclear binding studies performed in the presence of LXR or RXR antiserum confirmed that the consensus sequence found at nucleotides 1480-1510 is an LXR response element. This element also mediates a partial response to 9-c/s RA. All of the references cited herein, including patents and publications, are hereby incorporated in their entireties by reference. While the invention has been described with an emphasis upon prefeoed aspects of the invention, it will be readily apparent to those of ordinary skill in the art that variations of the preferred embodiments can be used and that it is intended that the invention can be practiced otherwise than is specifically described herein. Accordingly, the present invention includes all modifications encompassed within the spirit and scope of the invention as defined by the following claims.

Claims: What we claim is: 1. An isolated polynucleotide comprising SEQ ID NO: 3. 2. An isolated polynucleotide comprising nucleotides 1 - 1532, 1080- 1643, 1181- 1643, 1292-1643, 1394-1643, or 1394-1532 of SEQ ID NO: 3. 3. The polynucleotide of claim 2, wherein the polynucleotide comprises nucleotides 1394-1532 of SEQ ID NO: 3. 4. An isolated polynucleotide that hybridizes under stringent conditions to the polynucleotide of claim 1. 5. An isolated polynucleotide that hybridizes under stringent conditions to the polynucleotide of claim 2. 6. An isolated polynucleotide comprising a nucleotide sequence that is at least 80% identical to the polynucleotide of claim 1. 7. An isolated polynucleotide comprising a nucleotide sequence that is at least 80% identical to the polynucleotide of claim 2. 8. An isolated polynucleotide that is complementary to the polynucleotide of claim 1. 9. An isolated polynucleotide that is complementary to the polynucleotide of claim 2. 10. A composition comprising the polynucleotide of claim 1 and a suitable carrier. 1 1. A composition comprising the polynucleotide of claim 2 and a suitable carrier. 12. A recombinant vector comprising the polynucleotide of claim 1. 13. A recombinant vector comprising the polynucleotide of claim 2. 14. A host cell comprising the recombinant vector of claim 12. 15. A host cell comprising the recombinant vector of claim 13. 16. The recombinant vector of claim 12, further comprising at least one polynucleotide encoding a heterologous polypeptide. 17. The recombinant vector of claim 13, further comprising at least one polynucleotide encoding a heterologous polypeptide. 18. The recombinant vector of claim 16, wherein said heterologous polypeptide is selected from the group consisting of luciferase, β-galactosidase, chloramphenicol acetyl transferase transferase, and green fluorescent protein. 19. The recombinant vector of claim 18, wherein said heterologous polypeptide is luciferase. 20. The recombinant vector of claim 19, wherein said vector is pAPRl . 21. A method suitable for increasing cholesterol efflux from cells of a mammalian subject comprising the step of administering to the mammalian subject at least one ligand for a nuclear receptor in an amount sufficient to increase cholesterol efflux from said cells. 22. The method of claim 21 , wherein the ligand is selected from the group consisting of LXR, RXR, PPAR, FXR, and SXR ligands. 23. The method of claim 22, wherein said LXR ligand is selected from the group consisting of 20(S) hydroxycholesterol, 22(R) hydroxycholesterol, 24(S) hydroxycholesterol, 25- hydroxycholesterol, and 24(S), 25 epoxy cholesterol. 24. The method of claim 23, wherein said ligand is 20(S) hydroxycholesterol. 25. The method of claim 22, wherein said RXR ligand is selected from the group consisting of 9-c/s retinoic acid, retinol, retinal, all-trans retinoic acid, 13-c/s retinoic acid, acitretin, fenretinide, etretinate, CD 495, CD564, TTNN, TTNNPB, TTAB, LGD 1069. 26. The method of claim 25, wherein said ligand is 9-c/s retinoic acid. 27. The method of claim 22, wherein at least two ligands are administered to the mammalian subject. 28. The method of claim 27, wherein the increase in cholesterol efflux is at least 25- fold relative to the level of cholesterol efflux with either ligand alone. 29. The method of claim 27, wherein the ligands are 20(S) hydroxycholesterol and 9- cis retinoic acid. 30. A method suitable for increasing cholesterol efflux from cells of a mammalian subject comprising the step of administering to the mammalian subject an eicosanoid in an amount sufficient to increase cholesterol efflux in said cells. 31. The method of claim 30, wherein said eicosanoid is selected from the group consisting of prostaglandin E2, prostacyclin 12, and prostaglandin J2. 32. A method suitable for increasing the expression of ABCl in the cells of a mammalian subject comprising the step of administering to the mammalian subject a ligand for a nuclear receptor in an amount sufficient to increase the expression of ABCl in said cells. 33. The method of claim 32, wherein the ligand is selected from the group consisting of LXR, RXR, PPAR, FXR, and SXR ligands. 34. A method for screening a test compound for ABCl expression modulating activity comprising the steps of: operatively linking a reporter cDNA with an expression modulating portion of the mammalian ABC 1 gene to produce a recombinant reporter construct; transfecting the recombinant reporter construct into a population of host cells; assaying the level of reporter gene expression in a sample of the host cells; contacting the host cells with the test compound being screened; assaying the level of reporter gene expression in a sample of the host cells after contact with the test compound; and comparing the relative change in the level of reporter gene expression caused by exposure to the test compound, thereby determining the ABCl expression modulating activity. 35. The method of claim 34, wherein the expression modulating portion of the ABCl gene is the 5' flanking region. 36. The method of claim 35, wherein the 5' flanking region comprises SEQ ID NO: 3. 37. The method of claim 35, wherein the 5' flanking region comprises a polynucleotide comprising nucleotides 1-1532, 1080-1643, 1 181-1643, 1292-1643, 1394-1643, or 1394-1532 of SEQ ID NO: 3. 38. The method of claim 36, wherein the reporter cDNA is selected from the group consisting of luciferase, β-galactosidase, chloramphenicol acetyl transferase, and green fluorescent protein cDNA. 39. The method of claim 34, wherein the host cell is a mammalian cell. 40. The method of claim 38, wherein the recombinant reporter construct is pAPRl . 41. A kit suitable for screening a compound to determine the ABC 1 expression modulating activity of the compound comprising a recombinant reporter construct comprising a reporter cDNA operatively linked to an expression modulating portion of the mammalian ABCl gene in an amount sufficient for at least one assay and instructions for use. 42. The kit of claim 41 , wherein the expression modulating portion of the mammalian ABCl gene comprises the polynucleotide of claim 1. 43. The kit of claim 41 , wherein the expression modulating portion of the mammalian ABCl gene comprises the polynucleotide of claim 2. 44. The kit of claim 41 , wherein the reporter cDNA is selected from the group consisting of luciferase, β-galactosidase, chloramphenicol acetyl transferase, and green fluorescent protein cDNA. 45. The kit of claim 42, wherein the reporter cDNA is a luciferase cDNA. 46. The kit of claim 43, wherein the reporter cDNA is a luciferase cDNA. 47. The kit of claim 45, wherein the recombinant reporter construct is pAPRl . 48. The kit of claim 41 , further comprising means for detecting the reporter gene. 49. A method for identifying genes that are responsible for an inherited abnormality comprising the steps of: a. obtaining a first RNA sample from a first individual with the abnormality; b. obtaining a second RNA sample from a normal second individual; c. preparing mRNA from the first RNA sample to give a first mRNA sample; d. preparing mRNA from the second RNA sample to give a second mRNA sample; e. preparing a labeled first cDNA sample by reverse transcribing the first mRNA sample; f. preparing a gene expression aoay from the labeled first cDNA sample; g. preparing a labeled second cDNA sample by reverse transcribing the second mRNA sample; h. preparing a gene expression aoay from the labeled second cDNA sample; and i. probing the first gene expression array of the labeled first cDNA sample and probing the second gene expression array of the labeled second cDNA sample and identifying the variations in gene expression between the RNA cooesponding to the first cDNA sample and the RNA cooesponding to the second cDNA sample. 50. The method of claim 49 wherein the differences between the RNA coosponoding to the first cDNA sample and RNA cooesponding to the second cDNA sample are analyzed to identify a property selected from the group consisting of first RNA that is overexpressed in comparison to second RNA, first RNA that is underexpressed in comparison to the second RNA, first RNA that is absent in comparison to the second RNA, and first RNA that is present in comparison to second RNA. 51. The method of claim 49 wherein the abnormality is a negative abnormality. 52. The method of claim 49 wherein the individual is a mammal. 53. The method of claim 52 wherein the mammal is a human. 54. The method of claim 49 wherein at least one variation between the RNA cooesponding to the first cDNA sample and the RNA cooesponding to the second cDNA sample is evaluated to deteonine if it contributes to the abnormality. 55. The method of claim 49 wherein a third RNA sample is obtained from a an third individual having the same abnormality as first individual and the method of steps (a)-(g) are repeated substituting the third RNA sample for the first RNA sample and comparing the differences between the first and second and second and third to and identifying gene expression differences that are common to the abnormal individuals.

Inventors: GARVIN MICHAEL, LAWN RICHARD M, WADE DAVID, GARVIN, MICHAEL, LAWN, RICHARD, M., WADE, DAVID
Applicants: CV THERAPEUTICS INC, GARVIN MICHAEL, LAWN RICHARD M, WADE DAVID, CV THERAPEUTICS, INC., GARVIN, MICHAEL, LAWN, RICHARD, M., WADE, DAVID
Application year: 2000
Main classification: C12N15
IPC codes: A61K31, A61K35, A61K38, A61K39, A61K45, A61K48, A61P1, A61P3, A61P5, A61P9, A61P25, A61P43, C07K14, C07K16, C12N1, C12N5, C12N15, C12P21, C12Q1, C12R1, G01N33