Patent Publication Number: US-2010112586-A1

Title: Diagnosis of fetal abnormalities by comparative genomic hybridization analysis

Description:
CROSS-REFERENCE 
     This application claims the benefit of U.S. Provisional Application No. 60/804,818, filed Jun. 14, 2006, which application is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Analysis of specific cells can give insight into a variety of diseases. These analyses can provide non-invasive tests for detection, diagnosis and prognosis of diseases, thereby eliminating the risk of invasive diagnosis. For instance, social developments have resulted in an increased number of prenatal tests. However, the available methods today, amniocentesis and chorionic villus sampling (CVS) are potentially harmful to the mother and to the fetus. The rate of miscarriage for pregnant women undergoing amniocentesis is increased by 0.5-1%, and that figure is slightly higher for CVS. Because of the inherent risks posed by amniocentesis and CVS, these procedures are offered primarily to older women, i.e., those over 35 years of age, who have a statistically greater probability of bearing children with congenital defects. As a result, a pregnant woman at the age of 35 has to balance an average risk of 0.5-1% to induce an abortion by amniocentesis against an age related probability for trisomy 21 of less than 0.3%. 
     To eliminate the risks associated with invasive prenatal screening procedures, non-invasive tests for detection, diagnosis and prognosis of diseases, have been utilized. For example, maternal serum alpha-fetoprotein, and levels of unconjugated estriol and human chorionic gonadotropin are used to identify a proportion of fetuses with Down&#39;s syndrome, however, these tests are not one hundred percent accurate. Similarly, ultrasonography is used to determine congenital detects involving neural tube defects and limb abnormalities, but is useful only after fifteen weeks&#39; gestation. 
     The presence of fetal cells in maternal circulation offers the opportunity to develop a prenatal diagnostic that obviates the risk associated with today&#39;s invasive diagnostics procedures. However, fetal cells are rare as compared to the presence of maternal cells in the blood. Therefore, any proposed analysis of fetal cells to diagnose fetal abnormalities requires enrichment of fetal cells. Enriching fetal cells from maternal peripheral blood is challenging, time intensive and any analysis derived therefrom is prone to error. The present invention addresses these challenges. 
     The methods of the present invention allow for the detection of fetal cells and fetal abnormalities when fetal cells are present in a mixed population of cells, even when maternal cells dominate the mixture. 
     SUMMARY OF THE INVENTION 
     The present invention relates to methods for determining the presence of fetal cells and/or the presence of fetal abnormalities in a sample of a mixed cell population (e.g maternal cells and fetal cells). The method also provides for detecting the presence of one or more fetal alleles. In addition, the method can provide for the quantification of fetal DNA within a mixed sample. 
     Prior to analysis, a mixed sample can be enriched for fetal cells, and in some embodiments, fetal cells can constitute up to 50% of the cells in the sample. Samples can be derived from a variety of specimens including sweat, tears, ear flow, sputum, lymph, bone marrow suspension, lymph, urine, saliva, semen, vaginal flow, cerebrospinal fluid, brain fluid, ascites, milk, secretions of the respiratory, intestinal or genitourinary tracts fluid. Preferably, the samples are blood samples. 
     In some embodiments, determining involves hybridizing a DNA fragment in a mixed sample and a reference sample with one or more probes and comparing the hybridization level of the mixed sample to the hybridization level of the reference sample. Hybridization of DNA in the mixed sample and in the reference sample can be carried out simultaneously. 
     The DNA fragment(s) from the mixed sample and the DNA fragment(s) from the reference sample are identified by different labels. Examples of labels that can be used include chromophores, fluorescent moieties, enzymes, antigens, heavy metal, magnetic probes, dyes, phosphorescent groups, radioactive materials, chemiluminescent moieties, scattering or fluorescent nanoparticles, Raman signal generating moieties, or electrochemical detection moieties. 
     In some embodiments, the DNA fragments can be amplified prior to the hybridization reaction. Amplification can be attained using methods that include multiple displacement amplification (MDA), degenerate oligonucleotide primed PCR (DOP), primer extension pre-amplification (PEP), or improved-PEP (I-PEP). In some embodiments, DNA fragments can be amplified from autosomal or sex chromosomes. 
     The probes that are used in the hybridization reaction are bacterial artificial chromosome clones, metaphase chromosomes, PCR products, or synthesized DNA oligonucleotides. In some embodiments, the probes are oligonucleotide probes that are immobilized on a substrate. 
     The probes can be chosen to selectively hybridize to multiple regions within the same chromosome, or they may hybridize to regions on two or more chromosomes. When hybridization is to regions contained in two or more chromosomes, the reference sample is preferably a diluted mixed sample. In some embodiments, the regions to which the probes are selected to hybridize encompass a plurality of loci in which aneuploidy is suspected. 
     In some embodiments, kits are provided to perform some or all of the steps. These kits may include the devices and reagents needed to perform the cell enrichment and genetic analysis. 
    
    
     
       SUMMARY OF THE DRAWINGS 
         FIG. 1  illustrates a flow chart depicting the major steps involved in detecting a fetal abnormality using the methods described herein. 
         FIG. 2A-D  illustrate one embodiment of a size-based separation module. 
         FIGS. 3A-3C  illustrate one embodiment of an affinity separation module. 
         FIG. 4  illustrates one embodiment of a magnetic separation module. 
         FIG. 5  show the results of comparative genomic hybridization experiments. 
         FIG. 6  show the results of comparative genomic hybridization experiments. 
         FIGS. 7A-7D  illustrate various embodiments of the size-based separation module. 
         FIG. 8A-8B  illustrate cell smears of the product and waste fractions. 
         FIG. 9A-9F  illustrate isolated fetal cells confirmed by the reliable presence of male Y chromosome. 
         FIG. 10  illustrates trisomy 21 pathology in an isolated fetal nucleated red blood cell. 
         FIG. 11  illustrates the detection of single copies of a fetal cell genome by qPCR. 
         FIG. 12  illustrates detection of single fetal cells in binned samples by SNP analysis. 
         FIG. 13  illustrates a method of trisomy testing. The trisomy  21  screen is based on scoring of target cells obtained from maternal blood. Blood is processed using a cell separation module for hemoglobin enrichment (CSM-HE). Enriched cells are transferred to slides that are first stained and subsequently probed by FISH. Images are acquired, such as from bright field or fluorescent microscopy, and scored. The proportion of trisomic cells of certain classes serves as a classifier for risk of fetal trisomy  21 . Fetal genome identification can performed using assays such as: (1) STR markers; (2) qPCR using primers and probes directed to loci, such as the multi-repeat DYZ locus on the Y-chromosome; (3) SNP detection; and (4) CGH (comparative genome hybridization) array detection. 
         FIG. 14  illustrates assays that can produce information on the presence of aneuploidy and other genetic disorders in target cells. Information on aneuploidy and other genetic disorders in target cells may be acquired using technologies such as: (1) a CGH array established for chromosome counting, which can be used for aneuploidy determination and/or detection of intra-chromosomal deletions; (2) SNP/taqman assays, which can be used for detection of single nucleotide polymorphisms; and (3) ultra-deep sequencing, which can be used to produce partial or complete genome sequences for analysis. 
         FIG. 15  illustrates methods of fetal diagnostic assays. Fetal cells are isolated by CSM-HE enrichment of target cells from blood. The designation of the fetal cells may be confirmed using techniques comprising FISH staining (using slides or membranes and optionally an automated detector), FACS, and/or binning. Binning may comprise distribution of enriched cells across wells in a plate (such as a 96 or 384 well plate), microencapsulation of cells in droplets that are separated in an emulsion, or by introduction of cells into microarrays of nanofluidic bins. Fetal cells are then identified using methods that may comprise the use of biomarkers (such as fetal (gamma) hemoglobin), allele-specific SNP panels that could detect fetal genome DNA, detection of differentially expressed maternal and fetal transcripts (such as Affymetrix chips), or primers and probes directed to fetal specific loci (such as the multi-repeat DYZ locus on the Y-chromosome). Binning sites that contain fetal cells are then be analyzed for aneuploidy and/or other genetic defects using a technique such as CGH array detection, ultra deep sequencing (such as Solexa, 454, or mass spectrometry), STR analysis, or SNP detection. 
         FIG. 16  illustrates methods of fetal diagnostic assays, further comprising the step of whole genome amplification prior to analysis of aneuploidy and/or other genetic defects. 
     
    
    
     INCORPORATION BY REFERENCE 
     All publications and patent applications mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides systems, apparatuses, methods, and kits for detecting the presence and/or abnormalities of fetal cells in sample of mixed population (e.g., maternal cells and fetal cells). Abnormalities that can be detected include aneuploidy. In addition, the present invention provides methods to determine when there are insufficient fetal cells for a determination and report a non-informative case. In some embodiments, fetal cells in a sample are enriched prior to their detection and/or analysis. In some embodiments, detection and/or analysis may be performed directly on the sample without enrichment. 
     Aneuploidy means the condition of having less than or more than the normal diploid number of chromosomes. In other words, it is any deviation from euploidy. Aneuploidy includes conditions such as monosomy (the presence of only one chromosome or a pair in a cell&#39;s nucleus), trisomy (having three chromosomes of a particular type in a cell&#39;s nucleus), tetrasomy (having four chromosomes of a particular type in a cell&#39;s nucleus), pentasomy (having five chromosomes of a particular type in a cell&#39;s nucleus), triploidy (having three of every chromosome in a cell&#39;s nucleus), and tetraploidy (having four of every chromosome in a cell&#39;s nucleus). Birth of a live triploid is extraordinarily rare and such individuals are quite abnormal, however triploidy occurs in about 2-3% of all human pregnancies and appears to be a factor in about 15% of all miscarriages. Tetraploidy occurs in approximately 8% of all miscarriages. (http://www.emedicine.com/med/topic3241.htm). 
     Examples of fetal abnormalities that can be diagnosed by the methods of the present invention include, but are not limited to, trisomy 13, trisomy 18, trisomy 21 (Down Syndrome), Klinefelter Syndrome (XXY) and other irregular number of sex or autosomal chromosomes. Furthermore, the methods herein can distinguish maternal trisomy from paternal trisomy, and total aneuploidy from segmental aneuploidy. Additionally, the methods herein can be used to identify monoploidy, triploidy, tetraploidy, pentaploidy and other higher multiples of the normal haploid state. In some embodiments, the maternal or paternal origin of the fetal abnormality can be determined. 
     Aneuploidy means the condition of having less than or more than the normal diploid number of chromosomes. In other words, it is any deviation from euploidy. Aneuploidy includes conditions such as monosomy (the presence of only one chromosome of a pair in a cell&#39;s nucleus), trisomy (having three chromosomes of a particular type in a cell&#39;s nucleus), tetrasomy (having four chromosomes of a particular type in a cell&#39;s nucleus), pentasomy (having live chromosomes of a particular type in a cell&#39;s nucleus), triploidy (having three of every chromosome in a cell&#39;s nucleus), and tetraploidy (having lour of every chromosome in a cell&#39;s nucleus). Birth of a live triploid is extraordinarily rare and such individuals are quite abnormal, however triploidy occurs in about 2-3% of all human pregnancies and appears to be a factor in about 15% of all miscarriages. Tetraploidy occurs in approximately 8% of all miscarriages. (http://www.emedicine.com/med/topic3241.htm). 
     Segmental aneupolidy refers to changes in the copy number of intra-chromosomal regions. Normal diploid cells have two copies of each chromosome and thus two alleles of each gene or loci. Changes in the allele abundance for a particular chromosomal region may be indicative of a chromosomal rearrangement, such as a deletion, duplication or translocation event. 
       FIG. 1  illustrates an overview of one embodiment oldie present invention. 
     In step  100 , a sample containing (or suspected of containing) 1 or more fetal cells is obtained. Samples can be obtained from an animal suspected of being pregnant, pregnant, or that has been pregnant to detect the presence of a fetus or fetal abnormality. Such animal can be a human or a domesticated animal such as a cow, chicken, pig, horse, rabbit, dog, cat, or goat. Samples derived from an animal or human can include, e.g., whole blood, sweat, tears, ear flow, sputum, lymph, bone marrow suspension, lymph, urine, saliva, semen, vaginal flow, cerebrospinal fluid, brain fluid, ascites, milk, secretions of the respiratory, intestinal or genitourinary tracts fluid. 
     To obtain a blood sample, any technique known in the art may be used, e.g. a syringe or other vacuum suction device. A blood sample can be optionally pre-treated or processed prior to enrichment. Examples of pre-treatment steps include the addition of a reagent such as a stabilizer, a preservative, a tixant, a lysing reagent, a diluent, an anti-apoptotic reagent, an anti-coagulation reagent, an anti-thrombotic reagent, magnetic property regulating reagent, a buffering reagent, an osmolality regulating reagent, a pH regulating reagent, and/or a cross-linking reagent. 
     When a blood sample is obtained, a preservative such an anti-coagulation agent and/or a stabilizer can be added to the sample prior to enrichment. This allows for extended time for analysis/detection. Thus, a sample, such as a blood sample, can be enriched and/or analyzed under any of the methods and systems herein within 1 week, 6 days, 5 days, 4 days, 3 days, 2 days, 1 day, 12 hrs, 6 hrs, 3 hrs, 2 hrs, or 1 hr from the time the sample is obtained. 
     In some embodiments, a blood sample can be combined with an agent that selectively lyses one or more cells or components in a blood sample. For example, fetal cells can be selectively lysed releasing their nuclei when a blood sample including fetal cells is combined with deionized water. Such selective lysis allows for the subsequent enrichment of fetal nuclei using. e.g., size or affinity based separation. In another example platelets and/or enucleated red blood cells are selectively lysed to generate a sample enriched in nucleated cells, such as fetal nucleated red blood cells (fnRBC) and maternal nucleated blood cells (mnBC). The fnRBC&#39;s can subsequently be separated from the mnBC&#39;s using, e.g., affinity to antigen-i or magnetism differences in fetal and adult hemoglobin. 
     When obtaining a sample from an animal (e.g., blood sample), the amount can vary depending upon animal size, its gestation period, and the condition being screened. In some embodiments, up to 50, 40, 30, 20, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1 mL of a sample is obtained. In some embodiments, 1-50, 2-40, 3-30, or 4-20 mL of sample is obtained. In some embodiments, more than 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95 or 100 mL of a sample is obtained. 
     To detect fetal abnormality, a blood sample can be obtained from a pregnant animal or human within 36, 24, 22, 20, 18, 16, 14, 12, 10, 8, 6 or 4 weeks of gestation or even after the pregnancy has terminated. 
     In step  101 , a reference sample is obtained. The reference sample consists of substantially all or all maternal cells. In some embodiments, a reference sample is a maternal blood sample enriched for white blood cells (WBC&#39;s) such that it consists of substantially all or all maternal WBC&#39;s. In some embodiments, a reference sample is a diluted mixed sample wherein the dilution results in a sample free of fetal cells. For example, a maternal blood sample of 10-50 mL can be diluted by at least 2, 5, 10, 20, 50, or 100 fold to reduce the likelihood that it will include fetal cells. 
     In step  102 , when the sample to be tested or analyzed is a mixed sample (e.g. maternal blood sample), it is enriched for are cells or rare. DNA (e.g. fetal cells, fetal DNA or fetal nuclei) using one or more methods known in the art or disclosed herein. Such enrichment increases the ratio of fetal cells to non-fetal cells; the concentration of fetal DNA to non-fetal DNA; or the concentration of fetal cells in volume per total volume of the mixed sample. 
     In some embodiments, enrichment occurs by selective lysis as described above. For example, enucleated cells may be selectively lysed prior to subsequent enrichment steps or fetal nucleated cells may be selectively lysed prior to separation of the fetal nuclei from other cells and components in the sample. 
     In some embodiments, enrichment of fetal cells or fetal nuclei occurs using one or more size-based separation modules. Size-based separation modules include filtration modules, sieves, matrixes, etc., including those disclosed in International Publication Nos. WO 2004/113877, WO 2004/0144651, and US Application Publication No. 2004/011956. 
     In some embodiments, a size-based separation module includes one or more arrays of obstacles that form a network of gaps. The obstacles are configured to direct particles (e.g. cells or nuclei) as they flow through the array/network of gaps into different directions or outlets based on the particle&#39;s hydrodynamic size. For example, as a blood sample flows through an array of obstacles, nucleated cells or cells having a hydrodynamic size larger than a predetermined size, e.g., 8 microns, are directed to a first outlet located on the opposite side of the array of obstacles from the fluid flow inlet, while the enucleated cells or cells having a hydrodynamic size smaller than a predetermined size, e.g., 8 microns, are directed to a second outlet also located on the opposite side of the array of obstacles from the fluid flow inlet. 
     An array can be configured to separate cells smaller than a predetermined size from those larger than a predetermined size by adjusting the size of the gaps, obstacles, and offset in the period between each successive row of obstacles. For example, in some embodiments, obstacles and/or gaps between obstacles can be up to 10, 20, 50, 70, 100, 120, 150, 170, or 200 microns in length or about 2, 4, 6, 8 or 10 microns in length. In some embodiments, an array for size-based separation includes more than 100, 500, 1,000, 5,000, 10,000, 50,000 or 100,000 obstacles that are arranged into more than 10, 20, 50, 100, 200, 500, or 1000 rows. Preferably, obstacles in a first row of obstacles are offset from a previous (upstream) row of obstacles by up to 50% the period of the previous row of obstacles. In some embodiments, obstacles in a first row of obstacles are offset from a previous row of obstacles by up to 45, 40, 35, 30, 25, 20, 15 or 10% the period of the previous row of obstacles. Furthermore, the distance between a first row of obstacles and a second row of obstacles can be up to 10, 20, 50, 70, 100, 120, 150, 170 or 200 microns. A particular offset can be continuous (repeating for multiple rows) or non-continuous. In some embodiments, a separation module includes multiple discrete arrays of obstacles fluidly coupled such that they are in series with one another. Each array of obstacles has a continuous offset. But each subsequent (downstream) array of obstacles has an offset that is different from the previous (upstream) offset. Preferably, each subsequent array of obstacles has a smaller offset that the previous array of obstacles. This allows for a refinement in the separation process as cells migrate through the array of obstacles. Thus, a plurality of arrays can be fluidly coupled in series or in parallel, (e.g., more than 2, 4, 6, 8, 10, 20, 30, 40, 50). Fluidly coupling separation modules (e.g., arrays) in parallel allows for high-throughput analysis of the sample, such that at least 1, 2, 5, 10, 20, 50, 100, 200, or 500 mL per hour flows through the enrichment modules or at least 1, 5, 10, or 50 million cells per hour are sorted or flow through the device. 
       FIG. 2A-21 ) illustrate an example of a size-based separation module. Obstacles (which may be of any shape) are coupled to a flat substrate to form an array of gaps. A transparent cover or lid may be used to cover the array. The obstacles form a two-dimensional array with each successive row shifted horizontally with respect to the previous row of obstacles, where the array of obstacles directs component having a hydrodynamic size smaller than a predetermined size in a first direction and component having a hydrodynamic size larger that a predetermined size in a second direction. The flow of sample into the array of obstacles can be aligned at a small angle (lateral flow direction) with respect to a line-of-sight of the array. Optionally, the array is coupled to an infusion pump to perfuse the sample through the obstacles. The flow conditions of the size-based separation module described herein are such that cells are sorted by the array with minimal damage. This allows for downstream analysis of intact cells and intact nuclei to be more efficient and reliable. For enriching fetal cells from a mixed sample (e.g., maternal blood sample) the predetermined size of an array of obstacles can be between 4-10 microns, or 6-8 microns. 
     In one embodiment, a size-based separation module comprises an array of obstacles configured to direct fetal cells larger than a predetermined size to migrate along a line-of-sight within the array towards a first outlet or bypass channel leading to a first outlet, while directing cells and analytes smaller than a predetermined size through the array of obstacles in a different direction towards a second outlet. 
     A variety of enrichment protocols may be utilized although gentle handling of the cells is needed to reduce any mechanical damage to the cells or their DNA. This gentle handling also preserves the small number of fetal cells in the sample. Integrity of the nucleic acid being evaluated is an important feature to permit the distinction between the genomic material from the fetal cells and other cells in the sample. In particular, the enrichment and separation of the fetal cells using the arrays of obstacles produces gentle treatment which minimizes cellular damage and maximizes nucleic acid integrity permitting exceptional levels of separation and the ability to subsequently utilize various formats to very accurately analyze the genome of the cells which are present in the sample in extremely low numbers. 
     In some embodiments, enrichment of fetal cells occurs using one or more capture modules that selectively inhibit the mobility of one or more cells of interest. Preferable a capture module is fluidly coupled downstream to a size-based separation module. Capture modules can include a substrate having multiple obstacles that restrict the movement of cells or analytes greater than a predetermined size. Examples of capture modules that inhibit the migration of cells based on size are disclosed in U.S. Pat. Nos. 5,837,115, and 6,692,952. 
     In some embodiments, a capture module includes a two dimensional array of obstacles that selectively filters or captures cells or analytes having a hydrodynamic size greater than a particular gap size, e.g., predetermined size. Arrays of obstacles adapted for separation by capture can include obstacles having one or more shapes and can be arranged in a uniform or non-uniform order. In some embodiments, a two-dimensional array of obstacles is staggered such that each subsequent row of obstacles is offset from the previous row of obstacles to increase the number of interactions between the analytes being sorted (separated) and the obstacles. 
     Another example of a capture module is an affinity-based separation module. An affinity-based separation module capture analytes or cells of interest based on their affinity to a structure or particle as oppose to their size. One example of an affinity-based separation module is an array of obstacles that are adapted for complete sample flow through, but for the fact that the obstacles are covered with binding moieties that selectively bind one or more analytes (e.g., cell population) of interest (e.g., red blood cells, fetal cells, or nucleated cells) or analytes not-of-interest (e.g., white blood cells). Binding moieties can include e.g., proteins (e.g., ligands/receptors), nucleic acids having complementary counterparts in retained analytes, antibodies, etc. In some embodiments, an affinity-based separation module comprises a two-dimensional array of obstacles covered with one or more antibodies selected from the group consisting of: anti-CD71, anti-CD235a, anti-CD36, anti-carbohydrates, anti-selectin, anti-CD45, anti-GPA, and anti-antigen-i. 
       FIG. 3A  illustrates a path of a first analyte through an array of posts wherein an analyte that does not specifically bind to a post continues to migrate through the array, while an analyte that does bind a post is captured by the array.  FIG. 3B  is a picture of antibody coated posts.  FIG. 3C  illustrates coupling of antibodies to a substrate (e.g., obstacles, side walls, etc.) as contemplated by the present invention. Examples of such affinity-based separation modules are described in International Publication No. WO 2004/029221. 
     In some embodiments, a capture module utilizes a magnetic field to separate and/or enrich one or more analytes (cells) that has a magnetic property or magnetic potential. For example, red blood cells which are slightly diamagnetic (repelled by magnetic field) in physiological conditions can be made paramagnetic (attributed by magnetic field) by deoxygenation of the hemoglobin into methemoglobin. This magnetic property can be achieved through physical or chemical treatment of the red blood cells. Thus, a sample containing one or more red blood cells and one or more non-red blood cells can be enriched for the red blood cells by first inducing a magnetic property and then separating the above red blood cells from other analytes using a magnetic field (uniform or non-uniform). For example, a maternal blood sample can flow first through a size-based separation module to remove enucleated cells and cellular components (e.g., analytes having a hydrodynamic size less than 6 μms) based on size. Subsequently, the enriched nucleated cells (e.g., analytes having a hydrodynamic size greater than 6 pals) white blood cells and nucleated red blood cells are treated with a reagent, such as COD, N 2  or NaNO 2 , that changes the magnetic property of the red blood cells&#39; hemoglobin. The treated sample then flows through a magnetic field (e.g., a column coupled to an external magnet), such that the paramagnetic analytes (e.g., red blood cells) will be captured by the magnetic field while the white blood cells and any other non-red blood cells will flow through the device to result in a sample enriched in nucleated red blood cells (including fnRBC&#39;s). Additional examples of magnetic separation modules are described in U.S. application Ser. No. 11/323,971, filed Dec. 29, 2005 entitled “Devices and Methods for Magnetic Enrichment of Cells and Other Particles” and U.S. application Ser. No. 11/227,904, filed Sep. 15, 2005, entitled “Devices and Methods for Enrichment and Alteration of Cells and Other Particles”. 
     Subsequent enrichment steps can be used to separate the rare cells (e.g. fnRBC&#39;s) from the non-rare maternal nucleated red blood cells (non-RBC&#39;s). In some embodiments, a sample enriched by size-based separation followed by affinity/magnetic separation is further enriched for rare cells using fluorescence activated cell sorting (FACS) or selective lysis of a subset of the cells (e.g. fetal cells). In some embodiments, fetal cells are selectively bound to an anti-antigen i to separate them from the mnRBC&#39;s. In some embodiment, fetal cells or fetal DNA is distinguished from non-fetal cells or non-fetal DNA by forcing the rare cells (fetal cells) to become apoptotic, thus condensing their nuclei and optionally ejecting their nuclei. Rare cells such as fetal cells can be forced into apoptosis using various means including subjecting the cells to hyperbaric pressure (e.g. 4% CO 2 ). The condensed nuclei can be detected and/or isolated for further analysis using any technique known in the art including DNA gel electrophoresis, in situ labeling of DNA nicks (terminal deoxynucleotidyl transferase (TdT))-mediated dUTP in situ nick labeling (also known as TUNEL) (Gavrieli, Y., et al. J. Cell Biol 119:493-501 (1992)) and ligation of DNA strand breaks havine, one or two-base 3′ overhangs (Taq polymerase-based in situ ligation). (Didenko V., et al. J. Cell Biol. 135:1369-76 (1996)). 
     In some embodiments, when the analyte desired to be separated (e.g., red blood cells or white blood cells) is not ferromagnetic or does not have a magnetic property, a magnetic particle (e.g., a bead) or compound (e.g., Fe 3+ ) can be coupled to the analyte to give it a magnetic property. In some embodiments, a bead coupled to an antibody that selectively binds to an analyte of interest can be decorated with an antibody elected from the group of anti CD71 or CD75. In some embodiments a magnetic compound, such as Fe 3+ , can be couple to an antibody such as those described above. The magnetic particles or magnetic antibodies herein may be coupled to any one or more of the devices herein prior to contact with a sample or may be mixed with the sample prior to delivery of the sample to the device(s). In some embodiments, an uncoupled magnetic bead is mixed with an analyte desired to be separated (e.g., red blood cells or white blood cells). 
     Magnetic field used to separate analytes/cells in any of the embodiments herein can uniform or non-uniform as well as external or internal to the device(s) herein. An external magnetic field is one whose source is outside a device herein (e.g., container, channel, obstacles). An internal magnetic field is one whose source is within a device contemplated herein. An example of an internal magnetic field is one where magnetic particles may be attached to obstacles present in the device (or manipulated to create obstacles) to increase surface area for analytes to interact with to increase the likelihood of binding. Analytes captured by a magnetic field can be released by demagnetizing the magnetic regions retaining the magnetic particles. For selective release of analytes from regions, the demagnetization can be limited to selected obstacles or regions. For example, the magnetic field can be designed to be electromagnetic, enabling turn-on and turn-off off the magnetic fields for each individual region or obstacle at will. 
       FIG. 4  illustrates an embodiment of a device configured for capture and isolation of cells expressing the transferring receptor from a complex mixture. Monoclonal antibodies to CD71 receptor are readily available off-the-shelf and can be covalent)), coupled to magnetic materials, such as, but not limited to any conventional ferroparticle including but not limited to ferrous doped polystyrene and ferroparticles or Ferro-colloids (e.g., from Miltenyi or Dynal). The anti CD71 hound to magnetic particles is flowed into the device. The antibody coated particles are drawn to the obstacles (e.g., posts), floor, and walls and are retained by the strength of the magnetic field interaction between the particles and the magnetic field. The particles between the obstacles and those loosely retained with the sphere of influence of the local magnetic fields away from the obstacles are removed by a rinse. 
     One or more of the enrichment modules herein (e.g., size-based separation module(s) and capture module(s)) may be fluidly coupled in series or in parallel with one another. For example a first outlet from a separation module can be fluidly coupled to a capture module. In some embodiments, the separation module and capture module are integrated such that a plurality of obstacles acts both to deflect certain analytes according to size and direct them in a path different than the direction of analyte(s) of interest, and also as a capture module to capture, retain, or bind certain analytes based on size, affinity, magnetism or other physical property. 
     In any of the embodiments herein, the enrichment steps performed have a specificity and/or sensitivity ≧50, 60, 70, 80, 90, 95, 96, 97, 98, 99, 99.1, 99.2, 99.3, 99.4, 99.5, 99.6, 99.7, 99.8, 99.9 or 99.95% The retention rate of the enrichment module(s) herein is such that ≧50, 60, 70, 80, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, or 99.9% of the analytes or cells of interest (e.g., nucleated cells or nucleated red blood cells or nucleated from red blood cells) are retained. Simultaneously, the enrichment modules are configured to remove ≧50, 60, 70, 80, 85, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, or 99.9% of all unwanted analytes (e.g., red blood-platelet enriched cells) from a sample. 
     Any or all of the enrichment steps can occur with minimal dilution of the sample. For example, in some embodiments the analytes of interest are retained in an enriched solution that is less than 50, 40, 30, 20, 10, 9.0, 8.0, 7.0, 6.0, 5.0, 4.5, 4.0, 3.5, 3.0, 2.5, 2.0, 1.5, 1.0, or 0.5 fold diluted from the original sample. In some embodiments, any or all of the enrichment steps increase the concentration of the analyte of interest (fetal cell), for example, by transferring them from the fluid sample to an enriched fluid sample (sometimes in a new fluid medium, such as a buffer). The new concentration of the analyte of interest may be at least 2, 4, 6, 8, 10, 20, 50, 100, 200, 500, 1,000, 2,000, 5,000, 10,000, 20,000, 50,000, 100,000, 200,000, 500,000, 1,000,000, 2,000,000, 5,000,000, 10,000,000, 20,000,000, 50,000,000, 100,000,000, 200,000,000, 500,000,000, 1.000,000,000, 2,000,000,000, or 5,000,000,000 fold more concentrated than in the original sample. For example, a 10 times concentration increase of a first cell type out of a blood sample means that the ratio of first cell type/all cells in a sample is 10 times greater alter the sample was applied to the apparatus herein. Such concentration can take a fluid sample (e.g., a blood sample) of greater than 10, 15, 20, 50, or 100 mL total volume comprising rare components of interest, and it can concentrate such rare component of interest into a concentrated solution of less than 0.5, 1, 2, 3, 5, or 10 mL total volume. 
     The final concentration of fetal cells in relation to non-fetal cells alter enrichment can be about 1/10,000- 1/10, or 1/1,000, 1/100. In some embodiments, the concentration of fetal cells to maternal cells may be up to 1/1,000, 1/100, or 1/10 or as low as 1/100, 1/1,000 or 1/10,000. 
     Thus, detection and analysis of the fetal cells can occur even lithe non-fetal (e.g. maternal) cells are &gt;50%, 60%, 70%, 80%, 90%, 95%, or 99% of all cells in a sample. In some embodiments, fetal cells are at a concentration of less than 1:2, 1:4, 1:10, 1:50, 1:100, 1:1000, 1:10,000, 1:100,000, 1,000,000, 1:10,000,000 or 1:100,000,000 of all cells in a mixed sample to be analyzed or at a concentration of less than 1×10 −3 , 1×10 −4 , 1×10 −5 , 1×10 −6 , or 1×10 −6  cells/μL of the mixed sample. Over all, the number of fetal cells in a mixed sample, (e.g. enriched sample) has up to 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 30, 40, 50, 100 total fetal cells. 
     Enriched target cells (e.g., fnRBC) can be “binned” prior to analysis of the enriched cells ( FIGS. 15 &amp; 16 ). Binning is any process which results in the reduction of complexity and/or total cell number of the enriched cell output. Binning may be performed by any method known in the art or described herein. One method of binning the enriched cells is by serial dilution. Such dilution may be carried out using any appropriate platform (e.g., PCR wells, microliter plates). Other methods include nanofluidic systems which separate samples into droplets (e.g., BioTrove, Raindance, Fluidigm). Such nanofluidic systems may result in the presence of a single cell present in a nanodroplet. 
     Binning may be preceded by positive selection for target cells including, but not limited to affinity binding (e.g. using anti-CD71 antibodies). Alternately, negative selection of non-target cells may precede binning. For example, output from the size-based separation module may be passed through a magnetic hemoglobin enrichment module (MHEM) which selectively removes WBCs from the enriched sample. 
     For example, the possible cellular content of output from enriched maternal blood which has been passed through a size-based separation module (with or without further enrichment by passing the enriched sample through a MHEM) may consist of: 1) approximately 20 fnRBC; 2) 1,500 nmRBC; 3)-4,000-40,000 WBC; 4) 15×10 6  RBC. If this sample is separated into 100 bins (PCR wells or other acceptable binning platform), each bin would be expected to contain: 1) 80 negative bins and 20 bins positive for one tURBC; 2) 150 nmRBC; 3) 400-4,000 WBC; 4) 15×10 4  RBC. If separated into 10,000 bins, each bin would be expected to contain: 1) 9,980 negative bins and 20 bins positive for one fnRBC: 2) 8,500 negative bins and 1,500 bins positive for one mnRBC; 3)&lt;1-4 WBC; 4) 15×10 2  RBC. One of skill in the art will recognize that the number of bins may be increased depending on experimental design and/or the platform used for binning. The reduced complexity of the binned cell populations may facilitate further genetic and cellular analysis of the target cells. 
     Analysis may be performed on individual bins to confirm the presence of target cells (e.g. fnRBC) in the individual bin. Such analysis may consist of any method known in the art, including, but not limited to, FISH, PCR, STR detection, SNP analysis, biomarker detection, and sequence analysis ( FIGS. 15 &amp; 16 ). 
     Fetal Biomarkers 
     In some embodiments fetal biomarkers may be used to detect and/or isolate fetal cells, after enrichment or after detection of fetal abnormality or lack thereof. For example, this may be performed by distinguishing between fetal and maternal nRBCs based on relative expression of a gene (e.g., DYS1, DYZ, CD-71, ε- and ƒ-globin) that is differentially expressed during fetal development. In preferred embodiments, biomarker genes are differentially expressed in the first and/or second trimester. “Differentially expressed,” as applied to nucleotide sequences or polypeptide sequences in a cell or cell nuclei, refers to differences in over/under-expression of that sequence when compared to the level of expression of the same sequence in another sample, a control or a reference sample. In some embodiments, expression differences can be temporal and/or cell-specific. For example, for cell-specific expression of biomarkers, differential expression of one or more biomarkers in the cell(s) of interest can be higher or lower relative to background cell populations. Detection of such difference in expression of the biomarker may indicate the presence of a rare cell (e.g., fnRBC) versus other cells in a mixed sample (e.g., background cell populations). In other embodiments, a ratio of two or more such biomarkers that are differentially expressed can be measured and used to detect rare cells. 
     In one embodiment, fetal biomarkers comprise differentially expressed hemoglobins. Erythroblasts (nRBCs) are very abundant in the early fetal circulation, virtually absent in normal adult blood and by having a short, finite lifespan, there is no risk of obtaining fnRBC which may persist from a previous pregnancy. Furthermore, unlike trophoblast cells, fetal erythroblasts are not prone to mosaic characteristics. 
     Yolk sac erythroblasts synthesize ε-, ƒ-, γ- and α-globins, these combine to form the embryonic hemoglobins. Between six and eight weeks, the primary site of erythropoiesis shifts from the yolk sac to the liver, the three embryonic hemoglobins are replaced by fetal hemoglobin (HbF) as the predominant oxygen transport system, and ε- and ƒ-globin production gives way to γ-, α- and β-globin production within definitive erythrocytes (Peschle et al., 1985). HbF remains the principal hemoglobin until birth, when the second globin switch occurs and β-globin production accelerates. 
     Hemoglobin (Hb) is a heterodimer composed of two identical α globin chains and two copies of a second globin. Due to differential gene expression during fetal development, the composition of the second chain changes from ε globin during early embryonic development (1 to 4 weeks of gestation) to γ globin during fetal development (6 to 8 weeks of gestation) to β globin in neonates and adults as illustrated in (Table 1). 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Relative expression of ε, γ and β in maternal and fetal RBCs. 
               
            
           
           
               
               
               
               
            
               
                   
                 ε 
                 γ 
                 B 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 1 st  trimester 
                 Fetal 
                 ++ 
                 ++ 
                 − 
               
               
                   
                   
                 Maternal 
                 − 
                 +/− 
                 ++ 
               
               
                   
                 2 nd  trimester 
                 Fetal 
                 − 
                 ++ 
                 +/− 
               
               
                   
                   
                 Maternal 
                 − 
                 +/− 
                 ++ 
               
               
                   
                   
               
            
           
         
       
     
     In the late-first trimester, the earliest time that fetal cells may be sampled by CVS, fnRBCs contain, in addition to α globin, primarily ε and γ globin. In the early to mid second trimester, when amniocentesis is typically performed, fnRBCs contain primarily γ globin with some adult β globin. Maternal cells contain almost exclusively α and β globin, with traces of γ detectable in some samples. Therefore, by measuring the relative expression of the ε, γ and β genes in RBCs purified from γ maternal blood samples, the presence of fetal cells in the sample can be determined. Furthermore, positive controls can be utilized to assess failure of the FISH analysis itself. 
     In various embodiments, fetal cells are distinguished from maternal cells based on the differential expression of hemoglobins β, γ or ε. Expression levels or RNA levels can be determined in the cytoplasm or in the nucleus of cells. Thus in some embodiments, the methods herein involve determining levels of messenger RNA (mRNA), ribosomal RNA (rRNA), or nuclear RNA (nRNA). 
     In some embodiments, identification of fnRBCs can be achieved by measuring the levels of at least two hemoglobins in the cytoplasm or nucleus of a cell. In various embodiments, identification and assay is from 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15 or 20 fetal nuclei. Furthermore, total nuclei arrayed on one or more slides can number from about 100, 200, 300, 400, 500, 700, 800, 5000, 10,000, 100,000, 1,000,000, 2,000,000 to about 3,000,000. In some embodiments, a ratio for γ/β or ε/β is used to determine the presence of fetal cells, where a number less than one indicates that a fnRBC(s) is not present. In some embodiments, the relative expression of γ/β or ε/β provides a fnRBC index (“FNI”), as measured by γ or ε relative to β. In some embodiments, a FNI for γ/β greater than 5, 10, 15, 20, 25, 30, 35, 40, 45, 90, 180, 360, 720, 975, 1020, 1024, 1250 to about 1250, indicate that a fnRBC(s) is present. In yet other embodiments, a FNI for γ/β of less than about 1 indicates that a fnRBC(s) is not present. Preferably, the above FNI is determined from a sample obtained during a first trimester. However, similar ratios can be used during second trimester and third trimester. 
     In some embodiments, the expression levels are determined by measuring nuclear RNA transcripts including, nascent or unprocessed transcripts. In another embodiment, expression levels are determined by measuring mRNA, including ribosomal RNA. There are many methods known in the art for imaging (e.g., measuring) nucleic acids or RNA including, but not limited to, using expression arrays from Affymetrix, Inc. or Illumina, Inc. 
     RT-PCR primers can be designed by targeting the globin variable regions, selecting the amplicon size, and adjusting the primers annealing temperature to achieve equal PCR amplification efficiency. Thus TagMan probes can be designed for each of the amplicons with well-separated fluorescent dyes, Alexa fluor®-355 for ε, Alexa Fluor®-488 for γ, and Alexa Fluor-555 for β. The specificity of these primers can be first verified using ε, γ, and β cDNA as templates. The primer sets that give the best specificity can be selected for further assay development. As an alternative, the primers can be selected from two exons spanning an intron sequence to amplify only the mRNA to eliminate the genomic DNA contamination. 
     The primers selected can be tested first in a duplex format to verify their specificity, limit of detection, and amplification efficiency using target cDNA templates. The best combinations of primers can be further tested in a triplex format for its amplification efficiency, detection dynamic range, and limit of detection. 
     Various commercially available reagents are available for RT-PCR, such as One-step RT-PCR reagents, including Qiagen One-Step RT-PCR Kit and Applied Biosytems TaqMan One-Step RT-PCR Master Mix Reagents kit. Such reagents can be used to establish the expression ratio of ε, γ, and β using purified RNA from enriched samples. Forward primers can be labeled for each of the targets, using Alexa fluor-355 fore, Alexa fluor-488 for γ, and Alexa fluor-555 for β. Enriched cells can be deposited by cytospinning onto glass slides. Additionally, cytospinning the enriched cells can be performed after in sun RT-PCR. Thereafter, the presence of the fluorescent-labeled amplicons can be visualized by fluorescence microscopy. The reverse transcription time and PCR cycles can be optimized to maximize the amplicon signal:background ratio to have maximal separation of fetal over maternal signature. Preferably, signal:background ratio is greater than 5, 10, 50 or 100 and the overall cell loss during the process is less than 50, 10 or 5%. 
     Fetal Cell Analysis 
     In step  103 , pre-amplification is performed to ensure that sufficient fetal DNA is available. Such pre-amplification step involves a ratio-preserving amplification. Such amplification can be performed on genomic DNA derived from both mixed sample (maternal fetal cell sample) and reference sample (maternal only sample). This ratio preserving amplification minimizes errors associated with amplification, such as different amplification factors for the different nucleic acid fragments. Examples of amplification techniques that can be used include, but are not limited to, multiple displacement amplification (Gonzalez et al. Environ. Microbiol; 7(7):1024-8 (2005)), two-stage PCR amplification (Klein et al. PNAS (USA)  96 ; (8):4494-9 (1999)) and linear amplification such as in vitro transcription (Liu et al. BMC Genomics: 4(1); 19 (2003)). 
     To the extent that random amplification errors occur, they can be reduced by averaging the copy number or copy number ratios determined at different loci over a genomic region in which aneuploidy is suspected. For example, a microarray with 1000 oligo probes per chromosome could provide a chromosome copy number with error bars ˜√{square root over (1000)} times smaller than those from the determination based on a single probe. One can also perform probe averaging over the specific genomic region(s) suspected for aneuploidy (e.g. chromosome 13, 18, 21, or X or Y). For example, a common known segmental aneuploidy would be tested for by averaging the probe data only over that known chromosome region rather than the entire chromosome. These random errors can be reduced by using a large number of probes per chromosome (e.g. at least 500,000, 1 million, 2 million, 10 million or 20 million different probes per target chromosome). 
     In step  105 , amplified genomic DNA regions representing the entire genome or regions suspected of abnormal chromosome numbers (e.g. chromosome 13, 18, 21, or X). Comparative genomic hybridization (CGH) can be used to determine copy numbers of genes and chromosomes. DNA extracted from a biological sample is hybridized to immobilized reference genomic DNA which can be in the Corm of bacterial artificial chromosome (BAC) clones (Cheung, et al., 2005), or PCR products, or synthesized DNA obligos representing specific genomic sequence tags (Barrett, et al., 2004, Bignell, et al., 2004). Comparing the strength of hybridization of two different biological samples to the immobilized DNA segments gives a copy number ratio between the two samples. 
     In step  104 , genomic DNA nucleic acid fragments of interest from the mixed and a reference samples are amplified prior to performing CGH analysis. Amplification of nucleic acid fragments from the mixed sample and reference sample can occur by a variety of mechanisms, some of which may employ PCR. Examples of PCR techniques that can be used in the present invention include, but are not limited to, quantitative PCR, quantitative fluorescent PCR (QF-PCR), multiplex fluorescent PCR (MF-PCR), real time PCR (RT-PCR), single cell PCR, restriction fragment length polymorphism PCR (PCR-RFLP), PCR-RFLP/RT-PCR-RFLP, hot start PCR, Nested PCR, in situ polonony PCR, in situ rolling circle amplification (RCA), bridge PCR, picotiter PCR and emulsion PCR. Other suitable amplification methods include the ligase chain reaction (LCR), transcription amplification, self-sustained sequence replication, selective amplification of target polynucleotide sequences, consensus sequence primed polymerase chain reaction (CP-PCR), arbitrarily primed polymerase chain reaction (AP-PCR), degenerate oligonucleotide-primed PCR (DOP-PCR) and nucleic acid based sequence amplification (NABSA). Additional examples of amplification techniques are described in, U.S. Pat. Nos. 5,242,794, 5,494,810, 4,988,617 and 6,582,938. In some cases, the genomic DNA amplified is converted to single strands DNA fragments prior to performing comparative hybridization using any method known in the art. 
     In some embodiments, genomic DNA or nucleic acid fragments from a test sample and nucleic acid fragments from a control sample are mixed prior to performing CGH analysis. 
     In some embodiments, when two biological samples are being compared (e.g. mixed and reference samples) are hybridized to a single array or plurality of probes, the two different labels reversed and to average the two results—this technique reduces dye bias and is often referred to as ‘fluor reversed pair’. So, for example, if a first label is used for labeling genomic DNA from the mixed sample and a second label is used for labeling genomic DNA from the reference sample, the experiment is repeated with the labels reverse such that the genomic DNA from the mixed sample is labeled with the second label and vice versa. Examples of labels that can be used herein to label nucleic acid fragments include, but are not limited to, chromophores, fluorescent moieties, enzymes, antigens, heavy metal, magnetic probes, dyes, phosphorescent groups, radioactive materials, chemiluminescent moieties, scattering or fluorescent nanoparticles, Raman signal generating moieties, and electrochemical detection moieties. In some embodiments, the use of long probes, such as BAC clones, provides an analog averaging of these kinds of errors. Alternatively, a larger number of shorter oligo probes (e.g. more than 100, 200, 500, 1,000, 2,000, 5,000, 10,000, 20,000, or 50,000 per target chromosome) may be superior because errors associated with the creation of the probe features are better averaged out. 
     Differences in amplification and hybridization efficiency from sequence region to sequence region may be minimized by constraining the choices of probes (e.g. probes) so that they have similar melting temperatures and avoid sequences that tend to produce secondary structure. Also, although these effects are not truly ‘random’, they can be averaged out by averaging the results from a large number of probes. However, these effects may result in a systematic tendency for certain regions or chromosomes to have slightly larger signals than others, after reference probe averaging, which may mimic aneuploidy. When these particular biases are in common between the two samples being compared (e.g. mixed and reference), they divide out if the results are normalized. Thus, control genomic region(s) believed to have the same copy number in both samples yield a ratio of one. 
     In step  106 , results from hybridization are used to declare if there is an insufficient number fetal DNA to make a call, e.g. non-informative call, or if sufficient fetal cells are detected to declare if the fetal cells are normal or abnormal in then genotype. Examples of abnormal fetal genotypes include aneuploidy such as, monosomy of one or more chromosomes (X chromosome monosomy, also known as Turner&#39;s syndrome), trisomy of one or more chromosomes (13, 18, 21, and X), tetrasomy and pentasomy of one or more chromosomes (which in humans is most commonly observed in the sex chromosomes, e.g. XXXX, XXYY, XXXY, XYYY, XXXXX, XXXXY, XXXYY, XYYYY and XXYYY), triploidy (three of every chromosome, e.g. 69 chromosomes in humans), tetraploidy (four of every chromosome, e.g. 92 chromosomes in humans) and multiploidy. In some embodiments, an abnormal fetal genotype is a segmental aneuploidy. Examples of segmental aneuploidy include, but are not limited to, 1p36 duplication, dup(17)(p11.2p11.2) syndrome, Down syndrome, Pelizaeus-Mezbacther disease, dup(22)(q11.2q11.2) syndrome, and cat-eye syndrome. In some cases, an abnormal fetal genotype is due to one or more deletions of sex or autosomal chromosomes, which may result in a condition such as Cri-du-chat syndrome, Wolf-Hirschhorn, Williams-Beuren syndrome, Charcot-Marie-Tooth disease, Hereditary neuropathy with liability to pressure palsies, Smith-Magenis syndrome, Neurofibromatosis, Alagille syndrome, Velocardiofacial syndrome, DiGeorge syndrome, Steroid sulfatase deficiency, Kallmann syndrome, Microphthalmia with linear skin defects, Adrenal hypoplasia, Glycerol kinase deficiency, Pelizaeus-Merzbacher disease, Testis-determining factor on Y, Azospermia (factor a), Azospermia (factor b), Azospermia (factor c), or 1p36 deletion, In some embodiments, a decrease in chromosomal number results in an XO syndrome. 
     In steps  107 - 109 , a determination is made as to the presence or absence of fetal DNA in the mixed test sample. These steps are optional. The determination of the presence of fetal DNA needs to be one such that it correlates with the results from the CGH analysis described above. Thus, if fetal DNA is present in an amount that would be expected to produce an aneuploidy signal, if aneuploidy was in fact the result of the CGH analysis, then that result is further confirmed. 
     The presence of fetal DNA can be determined by detecting fetal-specific alleles using e.g. polymorphic regions such as short tandem repeat (STR) or single nucleotide polymorphism (SNP). Detection of fetal specific alleles or polymorphic regions can be done by any method know in the art as well as those described in U.S. application Ser. Nos. 11/763,426 and 11/763,133, entitled “Diagnosis of Fetal Abnormalities Using Polymorphisms Including Short Tandem Repeats” and “Use of Highly Parallel SNP Genotyping for Fetal Diagnosis,” respectively, which are herein incorporated by reference. 
     In step  107 , polymorphic sites of both mixed and reference samples are amplified using known methods. In some cases, multiple sites are amplified on a single chromosome. 
     In step  108 , the amplified polymorphic site(s) are used to detect fetal alleles. Methods that can be used to detect fetal alleles herein include, but are not limited to, gas chromatography, supercritical fluid chromatography, liquid chromatography, including partition chromatography, adsorption chromatography, ion exchange chromatography, size-exclusion chromatography, thin-layer chromatography, and affinity chromatography, electrophoresis, including capillary electrophoresis, capillary zone electrophoresis, capillary isoelectric focusing, capillary electrochromatography, micellar clectrokinetic capillary chromatography, isotachophoresis, transient isotachophoresis and capillary gel electrophoresis, microarrays, head arrays, high-throughput genotyping technology, and molecular inversion probes (MIPs). 
     In some embodiments, the DNA polymorphic sites are analyzed using CGH analysis (as shown by the dashed arrow in  FIG. 1 ). For example, DNA polymorphic sites could be analyzed using a DNA microarray (substrate coupled to a plurality of oligonucleotide probes). Amplicons corresponding to different alleles at polymorphic sites could be detected and distinguished on the same microarray, which could be possible for SNP sites. 
     In step  109 , a ratio of fetal/maternal DNA copies is determined. Thus ratio helps interpret the CGH results from step  105 . If the observed copy ratios are inconsistent with hypothesized aneuploidy ratios in the CGH analysis and the estimated fetal/maternal DNA fraction, then a declaration of aneuploidy is not be made even though the observed copy ratio was clearly different from unity. For example, if the estimated fetal/material ratio was 0.2 and the observed copy number ratio error bar was between 1.02 and 1.03, then this ratio would be inconsistent with the hypothesis of a fetal trisomy (which should show a ratio of 1.05 in this case (0.1×3+0.9×2)/(1.0×2)=1.05) even though the observed ratio is significantly different from unity. 
     Any of the steps described above can be performed using a computer program product that comprises a computer executable logic that is recorded on a computer readable medium. For example, the computer program can be used for determining the presence, absence and/or conditions associated with a fetus by performing analysis on data derived from array hybridizing. In particular, the computer executable logic can determine fetal/maternal ratio, analyze data from CGH, and provide an output reflective of an evaluation of a fetal abnormality. 
     The computer executable logic can work in any computer that may be any of a variety of types of general-purpose computers such as a personal computer, network server, workstation, or other computer platform now or later developed. In some embodiments, a computer program product is described comprising a computer usable medium having the computer executable logic (computer software program, including program code) stored therein. The computer executable logic can be executed by a processor, causing the processor to perform functions described herein. In other embodiments, some functions are implemented primarily in hardware using, for example, a hardware state machine. Implementation of the hardware state machine so as to perform the functions described herein will be apparent to those skilled in the relevant arts. The program can provide a method for determining a fetal abnormality by accessing data that reflects the hybridization of a probe to a DNA fragment in a mixed sample and in a reference sample, comparing the data, and providing an output reflecting the presence or absence of an abnormality. 
     In one embodiment, the computer executing the computer logic of the invention may also include a digital input device such as a scanner. The digital input device can provide information on CGH analysis and the polymorphic site analysis obtained according to method of the invention. For instance, the scanner can provide an image by detecting fluorescent, radioactive, or other emissions; by detecting transmitted, reflected, or scattered radiation; by detecting electromagnetic properties or characteristics; or by other techniques. Various detection schemes are employed depending on the type of emissions and other factors. The data typically are stored in a memory device in the form of a data file. 
     In one embodiment, the scanner may identify one or more labeled targets. For instance, in the CGH analysis described herein nucleic acid fragments from the test sample may be labeled with a first dye that fluoresces at a particular characteristic frequency, or narrow band of frequencies, in response to an excitation source of a particular frequency. The nucleic acid fragments from the control sample may be labeled with a second dye that fluoresces at a different characteristic frequency. The excitation sources for the second dye may, but need not, have a different excitation frequency than the source that excites the first dye, e.g., the excitation sources could be the same, or different, lasers. 
     In one embodiment, a human being may inspect a printed or displayed image constructed from the data in an image file and may identify the data (e.g. fluorescence from microarray) that are suitable for analysis according to the method of the invention. In another embodiment, the information is provided in an automated, quantifiable, and repeatable way that is compatible with various image processing and/or analysis techniques. 
     Another aspect of the invention includes kits containing the devices and reagents for detecting fetal abnormalities. Such kits may include any combinations of the disclosed devices and reagents. An exemplary kits provides the arrays for the size-based separation or enrichment and reagents for performing CGH analysis. These reagents may include probes for hybridizing to both fetal and non-fetal cells. 
     While preferred embodiments of the present invention have been shown and described herein, it will be obvious to those skilled in the art that such embodiments are provided by way of example only. Numerous variations, changes, and substitutions will now occur to those skilled in the art without departing from the invention. It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention. It is intended that the following claims define the scope of the invention and that methods and structures within the scope of these claims and their equivalents be covered thereby. 
     EXAMPLES 
     Example 1 
     Separation of Fetal Cord Blood 
       FIGS. 7A-7D  shows a schematic of the device used to separate nucleated cells from fetal cord blood. 
     Dimensions: 100 mm×28 mm×1 mm 
     Array design: 3 stages, gap size=18, 12 and 8 μm for the first, second and third stage, respectively. 
     Device fabrication: The arrays and channels were fabricated in silicon using standard photolithography and deep silicon reactive etching techniques. The etch depth is 140 μm. Through holes for fluid access are made using KOH wet etching. The silicon substrate was sealed on the etched face to form enclosed fluidic channels using a blood compatible pressure sensitive adhesive (9795, 3M, St Paul, Minn.). 
     Device packaging: The device was mechanically mated to a plastic manifold with external fluidic reservoirs to deliver blood and buffer to the device and extract the generated fractions. 
     Device operation: An external pressure source was used to apply a pressure of 2.0 PSI to the buffer and blood reservoirs to modulate fluidic delivery and extraction from the packaged device. 
     Experimental conditions: Human fetal cord blood was drawn into phosphate buffered saline containing Acid Citrate Dextrose anticoagulants. 1 mL of blood was processed at 3 mL/hr using the device described above at room temperature and within 48 hrs of draw. Nucleated cells from the blood were separated from enucleated cells (red blood cells and platelets), and plasma delivered into a buffer stream of calcium and magnesium-free Dulbecco&#39;s Phosphate Buffered Saline (14190-144, Invitrogen, Carlsbad, Calif.) containing 1% Bovine Serum Albumin (BSA) (A8412-100 mL, Sigma-Aldrich, St Louis, Mo.) and 2 mM EDTA (15575-020, Invitrogen, Carlsbad, Calif.). 
     Measurement techniques: Cell smears of the product and waste fractions ( FIG. 8A-8B ) were prepared and stained with modified Wright-Giemsa (WG 16, Sigma Aldrich, St. Louis, Mo.). 
     Performance: Fetal nucleated red blood cells were observed in the product fraction ( FIG. 8A ) and absent from the waste fraction ( FIG. 8B ). 
     Example 2 
     Isolation of Fetal Cells from Maternal blood 
     The device and process described in detail in Example 1 were used in combination with immunomagnetic affinity enrichment techniques to demonstrate the feasibility of isolating fetal cells from maternal blood. 
     Experimental conditions: blood from consenting maternal donors carrying male fetuses was collected into K 2 EDTA vacutainers (366643, Becton Dickinson, Franklin Lakes, N.J.) immediately following elective termination of pregnancy. The undiluted blood was processed using the device described in Example 1 at room temperature and within 9 hrs of draw. Nucleated cells from the blood were separated from enucleated cells (red blood cells and platelets), and plasma delivered into a buffer stream of calcium and magnesium-free Dulbecco&#39;s Phosphate Buffered Saline (14190-144, Invitrogen, Carlsbad, Calif.) containing 1% Bovine Serum Albumin (BSA) (A8412-100 mL, Sigma-Aldrich, St Louis, Mo.). Subsequently, the nucleated cell fraction was labeled with anti-CD71 microbcads (130-046-201, Miltenyi Biotech Inc., Auburn, Calif.) and enriched using the MiniMACS™ MS column (130-042-201, Miltenyi Biotech Inc., Auburn, Calif.) according to the manufacturer&#39;s specifications. Finally, the CD71-positive fraction was spotted onto glass slides. 
     Measurement techniques: Spotted slides were stained using fluorescence in situ hybridization (FISH) techniques according to the manufacturer&#39;s specifications using Vysis probes (Abbott Laboratories, Downer&#39;s Grove, Ill.). Samples were stained from the presence of X and Y chromosomes. In one case, a sample prepared from a known Trisomy  21  pregnancy was also stained for chromosome 21. 
     Performance: Isolation of fetal cells was confirmed by the reliable presence of male cells in the CD71-positive population prepared from the nucleated cell fractions ( FIG. 9A-9F ). In the single abnormal case tested, the trisomy  21  pathology was also identified ( FIG. 10 ). 
     Example 3 
     Confirmation of the Presence of Male Fetal Cells in Enriched Samples 
     Confirmation of the presence of a male fetal cell in an enriched sample is performed using 1PCR with primers specific for DYZ, a marker repeated in high copy number on the Y chromosome. After enrichment of fnRBC by any of the methods described herein, the resulting enriched fnRBC are binned by dividing the sample into 100 PCR wells. Prior to binning, enriched samples may be screened by FISH to determine the presence of any fnRBC containing an aneuploidy of interest. Because of the low number of fnRBC in maternal blood, only a portion of the wells will contain a single fnRBC (the other wells are expected to be negative for fnRBC). The cells are fixed in 2% Paraformaldehyde and stored at 4° C. Cells in each bin are pelleted and resuspended in 5 μl PBS plus 1p1 20 mg/ml Proteinase K (Sigma #P-2308). Cells are lysed by incubation at 65° C. for 60 minutes followed by inactivation of the Proteinase K by incubation for 15 minutes at 95° C. For each reaction, primer sets (DYZ forward primer TCGAGTGCATFCCATICCG; DYZ reverse primer ATGGAATGGCATCAAACGGAA; and DYZ Taqman Probe 6FAM-TGGCTGTCCATTCCA-MGBNFQ), TaqMan Universal PCR master mix. No AmpErase and water are added. The samples are run and analysis is performed on an ABI 7300: 2 minutes at 50° C., 10 minutes 95° C. followed by 40 cycles of 95° C. (15 seconds) and 60° C. (1 minute). Following confirmation of the presence of male fetal cells, further analysis of bins containing fnRBC is performed. Positive bins may be pooled prior to further analysis. 
       FIG. 15  shows the results expected from such an experiment. The data in  FIG. 15  was collected by the following protocol. Nucleated red blood cells were enriched from cord cell blood of a male fetus by sucrose gradient two Home Extractions (HE). The cells were fixed in 2% paraformaldehyde and stored at 4° C. Approximately 10×1000 cells were pelletal and resuspended each in 5 μl PBS plus 1 μl 20 mg/ml Proteinase K (Sigma HP-2308). Cells were lysed by incubation at 65° C. for 60 minutes followed by a inactivation of the Proteinase K by 15 minute at 95° C. Cells were combined and serially diluted 10-fold in PBS for 100, 10 and 1 cell per 6 μl final concentration were obtained. Six μl of each dilution was assayed in quadruplicate in 96 well format. For each reaction, primer sets (DYZ forward primer TCGAGTGCATIVCATTCCG; 0.9 μM DYZ reverse primer ATGGAATGGCATCAAACGGAA; and 0.5 μM DYZ TaqMan Probe 6FAM-TGGCTGTCCA-ITCCA-MGBNFQ), TaqMan Universal PCR master mix, No AmpErase and water were added to a final volume of 25 μl per reaction. Plates were run and analyzed on an ABI 7300: 2 minutes at 50° C., 10 minutes 95° C. followed by 40 cycles of 9.5″C (15 seconds) and 60° C. (1 minute). These results show that detection of a single fnRBC in a bin is possible using this method. 
     Example 4 
     Confirmation of the Presence of Fetal Cells in Enriched Samples by STR Analysis 
     Maternal blood is processed through a size-based separation module, with or without subsequent MHEM enhancement of fnRBCs. The enhanced sample is then subjected to FISH analysis using probes specific to the aneuploidy of interest (e.g., triploidy  13 , triploidy  18 , and XYY). Individual positive cells are isolated by “plucking” individual positive cells from the enhanced sample using standard micromanipulation techniques. Using a nested PCR protocol, STR marker sets are amplified and analyzed to confirm that the FISH-positive aneuploid cell(s) are of fetal origin. For this analysis, comparison to the maternal genotype is typical. An example of a potential resulting data set is shown in Table 2. Non-maternal alleles may be proven to be paternal alleles by paternal genotyping or genotyping of known fetal tissue samples. As can be seen, the presence of paternal alleles in the resulting cells, demonstrates that the cell is of fetal origin (cells #1, 2, 9, and 10). Positive cells may be pooled for further analysis to diagnose aneuploidy of the fetus, or may be further analyzed individually. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 STR locus alleles in maternal and fetal cells 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 STR 
                 STR 
                   
                   
                   
               
               
                   
                 locus 
                 locus 
                 STR locus 
                 STR locus 
                 STR locus 
               
               
                 DNA Source 
                 D14S 
                 D16S 
                 D8S 
                 F13B 
                 vWA 
               
               
                   
               
               
                 Maternal alleles 
                 14, 17 
                 11, 12 
                 12, 14 
                 9, 9 
                 16, 17 
               
               
                 Cell #1 alleles 
                   
                 8 
                   
                   
                 19 
               
               
                 Cell #2 alleles 
                 17 
                   
                 15 
               
               
                 Cell #3 alleles 
                   
                   
                 14 
               
               
                 Cell #4 alleles 
               
               
                 Cell #5 alleles 
                 17 
                 12 
                   
                 9 
               
               
                 Cell #6 alleles 
               
               
                 Cell #7 alleles 
                   
                   
                   
                   
                 19 
               
               
                 Cell #8 alleles 
               
               
                 Cell #9 alleles 
                 17 
                   
                 14 
                 7, 9 
                 17, 19 
               
               
                 Cell #10 alleles 
                   
                   
                 15 
               
               
                   
               
            
           
         
       
     
     Example 5 
     Confirmation of the Presence of Fetal Cells in Enriched Samples by SNP Analysis 
     Maternal blood is processed through a size-based separation module, with or without subsequent MHEM enhancement of fnRBCs. The enhanced sample is then subjected to FISH analysis using probes specific to the aneuploidy of interest (e.g., triploidy  13 , triploidy  18 , and XYY). Samples testing positive with FISH analysis are then binned into 96 microtiter wells, each well containing 15 μl of the enhanced sample. Of the 96 wells, 5-10 are expected to contain a single fnRBC and each well should contain approximately 1000 nucleated maternal cells (both WBC and mnRBC). Cells are pelleted and resuspended in 5 μl PBS plus 1 μl 20 mg/ml Proteinase K (Sigma #P-2308). Cells are lysed by incubation at 65° C. for 60 minutes followed by it inactivation of the Proteinase K by 15 minute at 95° C. 
     In this example, the maternal genotype (BB) and fetal genotype (AB) for a particular set of SNPs is known. The genotypes A and B encompass all three SNPs and diner from each other at all three SNPs. The following sequence from chromosome 7 contains these three SNPs (rs7795605, rs7795611 and rs7795233 indicated in brackets, respectively) (ATGCAGCAAGGCACAGACTAA[G/A]CAAGGAGA[G/C]GCAAAATITTC[A/G]TAGGGGAGAGAAATGGGTCAT T). 
     In the first round of PCR, genomic DNA from binned enriched cells is amplified using primers specific to the outer portion of the fetal-specific allele A and which flank the interior SNP (forward primer ATGCAGCAAGGCACAGACTACG; reverse primer AGAGGGGAGAGAAATGGGTCATT). In the second round of PCR, amplification using real time SYBR Green PCR is performed with primers specific to the inner portion of allele A and which encompass the interior SNP (forward primer CAAGGCACAGACTAAGCAAGGAGAG: reverse primer GGCAAAATITTCATAGGGGAGAGAAATGGGTCATT). 
     Expected results are shown in FIGURE Z. Here, six of the 96 wells test positive for allele A, confirming the presence of cells of fetal origin, because the maternal genotype (BB) is known and cannot lie positive for allele A. DNA from positive wells may be pooled for further analysis or analyzed individually. 
     Example 6 
     Comparative Genomic Hybridization (CGH) for Aneuploidy Results 
     Agilent Technologies commercial human CGH array and whole genome amplification procedure (based on multiple displacement amplification) were used to demonstrate the ability to detect aneuploidy in target cells resident in cell mixtures. The test sample was simulated with genomic DNA from a cell line with a triple-X chromosome, and the control sample was DNA from it normal (diploid-X) cell line. Differential (2-color) hybridization was performed with amplification products from: (1) the control DNA and (2) a mixture of 70% control DNA and 30% triple-X DNA. Hybridization ratios for the probes were log-averaged over each chromosome. Approximately 1800 probes were resident on the X chromosome in this microarray design.  FIGS. 5 and 6  show the results of these experiments. The error bars in  FIGS. 5 and 6  reflect one standard deviation expected error in the mean of the log 10  ratios for the probes over each chromosome. The number of genome copies (starting cells) was 100 for  FIGS. 5 and 10  for  FIG. 6 . It was found, as expected, that departures from unity ratio for the normal chromosomes tend to be larger as the starting DNA amounts decrease. In both figures the X aneuploidy is detected as a departure of several standard deviations, whereas the other chromosomes are not significantly different from unit ratio at a level or significance of two standard deviations. 
     In these experiments, the raw hybridization values actually showed larger errors, but these errors were consistent from experiment to experiment in terms of which chromosome regions tended to be biased high or low. When these systematic bias patterns were learned from a previous data set, and applied as a correction to the subject data set, the values shown in  FIGS. 5 and 6  were obtained. This adaptive correction was done using singular value decomposition of the chromosome-averaged biases over the set of experiments, and was applied to the value of all but Chromosome X. 
     Example 7 
     Fetal Diagnosis with CGH 
     Fetal cells or nuclei will be isolated as described in the enrichment section or as described in example 1 and 2. Comparative genomic hybridization (CGH) will be used to determine copy numbers of genes and chromosomes. DNA extracted from the enriched fetal cells will be hybridized to immobilized reference DNA which can be in the form of bacterial artificial chromosome (BAC) clones, or PCR products, or synthesized DNA oligos representing specific genomic sequence tags. Comparing the strength of hybridization fetal and maternal control cells to the immobilized DNA segments gives a copy number ratio between the two samples. To perform CGH effectively starting with small numbers of cells, the DNA from the enriched fetal cells can be pre-amplified according to standard methods described in the art. 
     A ratio-preserving amplification of the DNA will be done to minimize these errors; i.e. this amplification method will be chosen to produce as close as possible the same amplification factor for all target regions of the genome. Appropriate methods would include multiple displacement amplification, the two-stage PCR, and linear amplification methods such as in vitro transcription. 
     To the extent the amplification errors are random, their effect can be reduced by averaging the copy number or copy number ratios determined at different loci over a genomic region in which aneuploidy is suspected. For example, a microarray with 1000 oligo probes per chromosome could provide a chromosome copy number with error bars ˜sqrt(1000) times smaller than those from the determination based on a single probe. It is also important to perform the probe averaging over the specific genomic region(s) suspected for aneuploidy. For example, a common known segmental aneuploidy would be tested for by averaging the probe data only over that known chromosome region rather than the entire chromosome. Random errors could be reduced by a very large factor using DNA microarrays such as Affymetrix arrays that could have a million or more probes per chromosome. 
     In practice other biases will dominate when the random amplification errors have been averaged down to a certain level, and these biases in the CGH experimental technique must be carefully controlled. For example, when the two biological samples being compared are hybridized to the same array, it is helpful to repeat the experiment with the two different labels reversed and to average the two results—this technique of reducing the dye bias is called a ‘fluor reversed pair’. To some extent the use of long ‘clone’ segments, such as BAC clones, as the immobilized probes provides an analog averaging of these kinds of errors; however, a larger number of shorter oligo probes should be superior because errors associated with the creation of the probe features are better averaged out. 
     Differences in amplification and hybridization efficiency from sequence region to sequence region may be systematically related to DNA sequence. These differences can be minimized by constraining the choices of probes so that they have similar melting temperatures and avoid sequences that tend to produce secondary structure. Also, although these effects are not truly ‘random’, they will be averaged out by averaging the results from a large number of array probes. However, these effects may result in a systematic tendency for certain regions or chromosomes to have slightly larger signals than others, after probe averaging, which may mimic aneuploidy. When these particular biases are in common between the two samples being compared, they divide out if the results are normalized so that control genomic regions believed to have the same copy number in both samples yield a unity ratio. 
     Alter performing CGH analysis trisomy can be diagnosed by comparing the strength of hybridization fetal cells and maternal control cells to the immobilized DNA segments which would give a copy number ratio between the two samples. 
     In one method, DNA samples are obtained from the genomic DNA from enriched fetal cells and a maternal tissue sample that is substantially free of fetal cells (e.g. diluted maternal blood sample, tissue biopsy, etc.). These samples are digested with the Alu 1 restriction enzyme, such as (Promega, catalog II R6281) in order to introduce nicks into the genomic DNA (e.g. 10 minutes at 55° C. followed by immediately cooling to ˜32° C.). The partially digested sample is then boiled and transferred to ice. This is followed by Terminal Deoxynucleotidyl (TdT) tailing with dTTP at 37° C. for 30 minutes. The sample is boiled again after completion of the tailing reaction, followed by a ligation reaction wherein capture sequences, complementary to the poly T tail and labeled with a fluorescent dye, such as Cy3/green and Cy5/red, are ligated onto the strands. If fetal DNA is labeled with Cy3 then the maternal DNA is labeled with Cy5, and vice versa. The ligation reaction is allowed to proceed for 30 minutes at room temperature before it is stopped by the addition of 0.5M EDTA. The labeled DNAs are then purified from the reaction components using a cleanup kit, such as the Zymo DNA Clean and Concentration kit. The purified tagged DNAs are resuspended in a mixture containing 2× hybridization buffer, which contains LNA dT blocker, call thymus DNA, and nuclease free water. The mixture is vortexed at 14,000 RPM for one minute after the tagged DNA is added, then it is incubated at 95° C.-100° C. for 10 minutes. The Tagged DNA hybridization mixture, containing both labeled DNAs is then incubated on a glass hybridization slide, which has been prepared with human bacterial artificial chromosomes (BAC), such as the 32K array set. BAC clones covering at least 98% of the human genome are available from BACPAC Resources, Oakland Calif. 
     The slide will then incubated overnight (˜16 hours) in a dark humidified chamber at 52° C. The slide is then washed using multiple post hybridization washed. The BAC microarray is then imaged using an epifluorescence microscope and a CCD camera interfaced to a computer. Analysis of the microarray images is performed using analysis software, such as the GenePix Pro 4.0 software (Axon Instruments. Foster City Calif.). For each spot the median pixel intensity minus the median local background for both dyes is used to obtain a test over reference gene copy number ratio. Data normalization is performed per array subgrid using lowess curve fitting with a smoothing factor of 0.33. To identify imbalances the MATLAB toolbox CGH plotter is applied, using moving mean average over three clones and limits of log2&gt;0.2. Classification as gain or loss is based on (1) identification as such by the CGH plotter and (2) visual inspection of the log2 ratios. In general, log2 ratios&gt;0.5 in at least four adjacent clones will be considered to be deviating. Ratios of 0.5-1.0 will be classified as duplications/hemizygous deletions; ratios &gt;1 will be classified as amplifications/homozygous deletions. All normalizations and analyses are carried out using commercially available analysis software, such as the BioArray Software Environment database. Regions of the genome that are either gained or lost in the fetal cells are indicated by the fluorescence intensity ratio profiles. Thus, in a single hybridization it is possible to screen the vast majority of chromosomal sites that may contain genes that are either deleted or amplified in the fetal cells 
     The sensitivity of CGH in detecting gains and losses of DNA sequences is approximately 0.2-20 Mb. For example, a loss of a 200 kb region should be detectable under optimal hybridization conditions. Prior to CGH hybridization, DNA can be universally amplified using degenerate oligonucleotide-primed PCR (DOP-PCR), which allows the analysis of, for example, rare fetal cell samples. The latter technique requires a PCR pre-amplification step. 
     Primers used for DOP-PCR have defined sequences at the 5′ end and at the 3′ end, but have a random hexamer sequence between the two defined ends. The random hexamer sequence displays all possible combinations of the natural nucleotides A, G, C, and T. DOP-PCR primers are annealed at low stringency to the denatured template DNA and hybridize statistically to primer binding sites. The distance between primer binding sites can be controlled by the length of the defined sequence at the 3′ end and the stringency of the annealing conditions. The first five cycles of the DOP-PCR thermal cycle consist of low stringency annealing, Mowed by a slow temperature increase to the elongation temperature, and primer elongation. The next thirty-five cycles use a more stringent (higher) annealing temperature. Under the more stringent conditions the material which was generated in the first five cycles is amplified preferentially, since the complete primer sequence created at the amplicon termini is required for annealing. DOP-PCR amplification ideally results in a smear of DNA fragments that are visible on an agarose gel stained with ethidium bromide. These fragments can be directly labelled by ligating capture sequences, complementary to the primer sequences and labeled with a fluorescent dye, such as Cy3/green and Cy5red. Alternatively the primers can be labelled with a florescent dye, in a manner that minimizes steric hindrance, prior to the amplification step.