Abstract:
Disclosed are methods for detecting in situ the presence of a target sequence in a substantially double-stranded nucleic acid segment, which comprises: a) contacting in situ under conditions suitable for hybridization a substantially double-stranded nucleic acid segment with a detectable third strand, said third strand being capable of hybridizing to at least a portion of the target sequence to form a triple-stranded structure, if said target sequence is present; and b) detecting whether hybridization between the third strand and the target sequence has occured.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a 371 of PCT/US98/23765 filed Nov. 10, 1998, which claims benefit of U.S. Ser. No. 60/064,997 filed Nov. 10, 1997. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     The present work was supported by grants from the National Institutes of Health (GM42936) and the Department of Energy (DE-FG02-96ER62202). 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to methods for identifying and classifying nucleic acids by means of in situ hybridization of a third-strand probe to a duplex DNA target. 
     2. Description of Related Art 
     A list of the references referred to by number herein is found at the end of the Detailed Description of the Invention herein. 
     Oligodeoxy- and ribo-nucleotide ‘third strands’ bind in sequence- and polarity-specific alignment within the major groove of cognate purine-rich•pyrimidine-rich nucleic acid duplex target sequences (1). A third-strand binding code has been elucidated (2) that encompasses triplexes of various structural motifs, of which the best characterized is the pyrlmidine-parallel (Y:R•Y) with T:A•T and C + :G•C triplets. The fidelity of third-strand binding to cognate targets of equal and lesser length, targets contained within large DNA molecules, and targets with inverted base pairs, i.e., R•Y→Y•R, is well documented (3). Optimal solution conditions for third-strand binding, i.e., pH, counterionic strength, and temperature, can be manipulated to induce third-strand dissociation or prevent association. While certain triplet mismatches are moderately tolerated within the Y:R•Y motif (4), e.g., T:C•G and G:T•A, others are especially destabilizing. Such destabilization provides temperature as a variable to selectively favor, under in vitro and demonstrated in situ conditions, desired triplexes over less perfect ones, as required in the large non-specific duplex DNA background of the human genome, while maintaining essentially constant pH and ionic strength. 
     The use of third strands for sequence-specific recognition of human genomic DNA has been exploited for the development of potential anti-gene therapeutic agents (5) and artificial endonucleases (6). By virtue of the present invention, this approach is extended to in situ hybridization for the detection and analysis of nucleic acids. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method for detecting in situ the presence of a target sequence in a substantially double-stranded nucleic acid segment, which comprises: 
     a) contacting in situ under conditions suitable for hybridization a substantially double-stranded nucleic acid segment with a detectable third strand, said third strand being capable of hybridizing to at least a portion of the target sequence to form a triple-stranded structure, if said target sequence is present in the nucleic acid segment; and 
     b) detecting whether hybridization between the third strand and the target sequence has occurred. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A depicts the organization of chromosome 17 ∝- satellite DNA. A tandem array of the chr.17 2.7 kb EcoRI (E) higher-order repeat, single higher-order repeat, and the sixteen 171 bp monomer units that comprise it. Gray rectangles represent the unique 16 bp third-strand target sequence within each monomer sixteen. 
     FIG. 1B depicts the 16-nt third strand [SEQ ID NO.2] and its 16 bp ∝-satellite target [SEQ ID NO.76] and [SEQ ID NO.77]. Third strand T and m 5 C +  residues are bound via Hoogsteen-hydrogen bonds (•) to the purine residues of their Watson-Crick (|) target [SEQ ID NO.76]. The boxed sequence shows the upstream ∝-satellite DNA T residues capable of forming photoadducts with the psoralen (4′-hydroxymethyl-4,5′,8-trimethylpsoralen, HMT) moiety of the dual-modified third strand. Ps=psoralen; Bi=biotin. 
     FIG. 2A is an autoradiograph showing the specificity of third-strand binding to supercoiled (sc) p17H8. Gel purified  32 P-end-labeled 5′-Tm 5 C-3′ (8×10 6  cpm/pmol) and 0.25 μg of 2×CsCl p17H8 or pMJ1 were mixed at 1:1 oligomer:plasmid molar ratio in 1×triplex cocktail±deionized formamide (0.25 or 2.5%) and yeast RNA (1 ng) at pH 6.0 and 23° C. for ˜20 hrs to attain equilibrium. 25 μl rxn samples were electrophoresed on a 0.8% SeaKem Gold (FMC) agarose gel overnight at 23° C. and 1.3 V/cm with buffer recirculation. Pre-flashed film (Dupont Reflection) was exposed to the dried gel at 4° C. for 15 min. Control lanes 1 and 8 contain  32 P-third strand alone, lanes 2-4  32 P-third strand and p17H8, and lanes 5-7  32 P-third strand and pMJ1 (lacking target). 
     FIG. 2B is an autoradiograph showing rate-dependent third strand binding to sc p17H8 in the presence of ˜2,400 unsorted human metaphase chromosomes (1-22, XY). Gel purified  32 P-end-labeled 5′-Tm 5 C-3′ (0.5×10 6  cpm/pmol) and 0.25 μg of 2×CsCl p17H8 were present at a 2:1 oligomer:plasmid molar ratio. 25 μl reactions were carried out in 1×triplex cocktail at pH 6.0 and 23° C. After 1.0, 2.5, and 5.0 hr incubations, the reactions were centrifuged at 3,000 rpm and 4° C. for 30 minutes to pellet the chromosomes. The supernatants were electrophoresed as described above, and film exposure was for 7 hr. Lane 1 contains  32 P-third strand and p17H8, but no chromiosomes. Lanes 2A, 3A, and 4A represent reactions in which chromosomes,  32 P-third strand, and p17H8 were added together, while for lanes 2B, 3B and 4B, chromosomes and  32 P-third strand were preincubated together for 1 hr prior to adding p17H8. 
     FIG. 3 is a standard non-denatured metaphase spread and interphase nucleus prepared for TISH, but “hybridized” with a D17Z1-specific ds probe using a conventional FISH protocol (Oncor, Inc.). Non-denatured spreads and nuclei were hybridized at 37° C. for 1 hr with 5 ng of a denatured 200 bp biotin-11dUTP nick-translated probe (Oncor, Inc.). No fluorescent signals are visible after FITC-avidin-based detection. The chromosomes are stained with PI. 
     FIGS. 4A and 4B are TISH images of a non-denatured (A) metaphase spread and (B) interphase nuclei hybridized wit the 16-nt chr. 17 ∝-satellite-specific third strand 5′Pso-Tm 5 C-Bio-3′. FITC-avidin-based detection clearly identifies two centromere-specific fluorescent spots on anatomically homologous metaphase chromosomes that are the expected size for chr.17. Each interphase nucleus shows two fluorescent spots. 
     FIGS. 5A-5C are TISH images of (A) a non-denatured mouse×human chr. 17 metaphase spread and (B) nuclei hybridized with the chr. 17 ∝-satellite-specific third strand 5′-Pso-Tm 5 C-Bio-3′. FITC-avidin-based detection is specific for the single human chr.17s. FIG. 5C is a TISH image showing a competition assay that demonstrates specificity of third-strand binding to chr.17 ∝-satellite in human metaphase spreads. A 25:1 molar ratio of 5′-Tm 5 C-3′ to 5′-Pso-Tm 5 C-Bio 3′ effectively competes out the dual modified third-strand. The white bar at the bottom left of each micrograph represents 10 microns. 
     FIGS. 6A-6D are TISH and FISH images of metaphase spreads and interphase nuclei from two individuals differing in their TISH-based chr.17 fluorescent signal patterns. The two haplotypes for third-strand binding, +17 and −17, are shown in two combinations, (A) +17/+17 and (C) +17/−17. FIGS.  6 (B) and  6 (D) are their corresponding +17/+17 FISH images. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Third Strand 
     The third strand is a synthetic or naturally occurring oligonucleotide capable of binding with specificity to a predetermined target region of a double-stranded nucleic acid molecule to form a triple-stranded structure. The third strand may bind solely to one strand of the native nucleic acid molecule, or may bind to both strands at different points along its length. 
     Preferably, the oligonucleotide probe is a single-stranded DNA molecule between about 7 and about 50, most preferably between about 10 and about 23 nucleotides in length. Its base composition can be homopurine, homopyrimidine, or a mixture of purines and pyrimidines. The third-strand binding code and preferred conditions under which a triple-stranded helix will form are well known to those skilled in the art (U.S. Pat. No. 5,422,251; Beal and Dervan,  Science  251: 1360 (1991); Beal and Dervan,  Nucleic Acids Res ., 20:2773 (1992); Broitman and Fresco,  Proc. Natl. Acad. Sci. USA , 84:5120 (1987); Fossella, et al.,  Nuc. Acids Res . 21:4511 (1993); Letai, et al.,  Biochemistry  27:9108 (1988); Sun, et al.,  Proc. Natl. Acad. Sci. USA  86:9198 (1989)). Briefly, adenosine, uridine, thymidine and inosine in the third strand will bind to adenosine in the duplex, and guanosine, cytidine and inosine in the third strand will bind to guanosine in the duplex. The third strand need not be perfectly complementary (in the binding code sense, not in the Watson-Crick sense) to the duplex, but may be substantially complementary. In general, by substantially complementary is meant that one mismatch is tolerable in about every 10 base pairs. 
     The third strand may have a natural phosphodiester backbone or may be comprised of other backbone chemical groups or mixtures of chemical groups which do not prevent the triple-stranded helix from forming. These alternative chemical groups include phosphorothioates, methylphosphonates, peptide nucleic acids (PNAs), and others known to those skilled in the art. Preferably, the third-strand backbone is phosphodiester. 
     The third strand may also comprise one or more modified sugars, which would be known to those skilled in the art. An example of such a sugar includes α-enantiomers. 
     The third strand may also incorporate one or more synthetic bases if such is necessary or desirable to improve third-strand binding. Examples of synthetic base design and the bases so designed are found in the co-pending U.S. application Ser. No. 08/473,888 of Fresco, et al. entitled “Residues for Binding Third Strands to Complementary Nucleic Acid Duplexes of any Base-Pair Sequence”, filed Jun. 7, 1995, and published as WO/9641009, the contents of which are incorporated herein by reference. 
     If it is necessary to protect the third strand from nucleases resident in the target cells, the third strand may be modified with one or more protective groups. In a preferred embodiment, the 3′ and 5′ ends may be capped with a number of chemical groups known to one of ordinary skill, such as alkyl amines, acridine, cholesterol, etc. In another embodiment, the third strand may be protected from exonucleases by circularization. 
     Label 
     The third strand should include a reporter group which has a physical or chemical characteristic which can be measured or detected by appropriate detector systems or procedures, which will allow detection of triplex formation. Detectability can be provided by such characteristics as color change, luminescence, fluorescence or radioactivity, or it can be provided by the ability of the reporter group to serve as a ligand recognition site. Such characteristics can be measured or detected, for example, by the use of conventional calorimetric, spectrophotometric, fluorometric or radioactivity sensing instrumentation, or by visual inspection, e.g., microscopic observation. 
     The interactions which can be usefully initiated by the reporter group include appropriately specific and selective interactions productive of groups or complexes which are themselves detectable, for example, by colorimetric, spectrophotometric, fluorometric or radioactive detection procedures. Such interactions can take the form of protein-ligand, enzyme-substrate, antibody-antigen, carbohydrate-lectin, protein-cofactor, protein-effector, nucleic acid-nucleic acid, and nucleic acid-ligand interactions. Examples of such ligand-ligand interactions include fluorescein-antifluorescein antibody, dinitrophenyl-dinitrophenyl antibody and biotin-avidin. Either one of each of such ligand pairs may serve as a ligand recognition type reporter group. Preferred reporter groups of the present invention include, for example, biotin, fluorescein, digoxigenin, phenyloxazolone, tetramethyl rhodamine, Texas Red, and BODIPY (contains the 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene unit). 
     General methods for joining the reporter group to the third strand are well known to those skilled in the art. Examples of those methods may be found, for example, in U.S. Pat. Nos. 4,711,955 and 5,684,142. 
     Target 
     The substantially double-stranded nucleic acid segment may comprise any combination of naturally occurring nucleic acid types, such as two DNA strands, two RNA strands, or a DNA-RNA hybrid. By “substantially double-stranded” is meant that the two strands which make up the nucleic acid segment need not be hybridized to each other along their entire length. It is preferred that the target sequence to which the third strand binds is completely double-stranded, but a degree of non-binding between the two strands of the target sequence is tolerable, so long as it does not exceed about 1 mismatch in 10 base pairs. 
     The target sequence may be any sequence for which detection is desired, and may be anywhere along the length of DNA or RNA found in a cell, including α-satellite regions, other satellite regions, non-satellite heterochromatin, or euchromatin regions. The target sequence may be within the chromosomal region of a eukaryotic species, particularly organisms of commercial significance in agriculture, or the double-stranded stage of a virus. 
     The in situ hybridization may take place in environments known to those skilled in the art, and are in most respects similar to those used for conventional in situ hybridization on denatured nucleic acids, although according to the present invention the target sequence is not denatured. The present hybridization may be performed on metaphase spreads and interphase nuclei, the preparation of which would be readily apparent to those skilled in the art. The major difference from conventional in situ hybridization is that in performance of the present invention, the hybridization conditions (e.g., temperature, pH,. salt concentration, etc.) are non-denaturing and should be such that, as described above, the target nucleic acid segment is substantially double-stranded. 
     It is preferred that the target sequence contain a purine-rich segment on one strand and a complementary pyrimidine-rich segment on the other strand. A stretch of at least ten consecutive purine bases is particularly preferred. 
     Binding Enhancement 
     The oligonucleotide may be modified to include a moiety which will enhance the binding of the third strand to the target sequence. Such a moiety may be positioned anywhere along the length of the third strand. Suitable moieties include well known DNA-binding, cross-linking or intercalating agents such as psoralen, acridine, coralyne, etc. The binding moiety is often incorporated into the oligonucleotide during its synthesis. For example, commercially available compounds such as psoralen C2 phosphoroamidite (Glen Research, Sterling Va.) are inserted into a specific location within an oligonucleotide sequence in accordance with the methods of Takasugi, et al.,  Proc. Natl. Acad. Sci USA , 88:5602 (1991); Gia, et al.,  Biochemistry  31:11818 (1992); Giovannangeli, et al.,  Proc. Natl. Acad. Sci. USA , 89:8631 (1992), all of which are incorporated by reference herein. The binding moiety may also be attached to the oligonucleotide through a linker, such as sulfo-m-maleimidonbenzoly-N-hydroxysuccinimide ester (sulfo-MBS, Pierce Chemical Company, Rockford Ill.) in accordance with the methods of Liu, et al.,  Biochem . 18:690 (1979) and Kitagawa and Ailawa,  J. Biochem . 79:233 (1976), both of which are incorporated by reference herein. 
     Utility 
     Third-strand in situ hybridization (TISH) complements and extends denaturant Watson-Crick-based FISH (7) technology to permit molecular cytogenetic and biochemical studies of non-denatured metaphase and interphase fixed chromosomes and chromatin, though it is not restricted to such uses. In Table 1 below, we have identified unique multicopy α-satellite third-strand target sequences in 22 of 24 human chromosomes, making centromere-specific chromosome identification by TISH applicable to virtually all human chromosomes. In the Table, third-strand target sequences (Column 2) within the ∝-Satellite DNA of a particular human chromosome type or chromosome group, e.g., 1, 15, 16 (Column 1) are given. Numbers after the decimal indicate that different third-strand target sequences are shared within a group or are unique to a single chromosome. Group (1, 15, 16) has three target sequences. Chromosome 3 contains four uniquely different target sequences. 
     
       
         
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Chromosome 
                   
               
               
                 Type or Group 
                 Target Sequence 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1.1 
                 AGTAAAGGAA AGAA 
                 [SEQ ID NO:3] 
               
               
                 1, 15, 16.1 
                 TATTTCCTTT TCTCGCTT 
                 [SEQ ID NO:4] 
               
               
                 1, 15, 16.2 
                 GAATGAAAAG GAAAG 
                 [SEQ ID NO:5] 
               
               
                 1, 15, 16.3 
                 TTTCCTTTTC TCGCTTT 
                 [SEQ ID NO:6] 
               
               
                 1, 5, 19.1 
                 AAAGGTAGAA AAGGAAATA 
                 [SEQ ID NO:7] 
               
               
                 1, 5, 19.2 
                 AAAGGCAGAA AAGGAAATA 
                 [SEQ ID NO:8] 
               
               
                 1, 5, 19.3 
                 CTTTTCTTTT TCATTC 
                 [SEQ ID NO:9] 
               
               
                 2.1 
                 CCTTTCTTTT GAGAGAGCAG 
                 [SEQ ID NO:10] 
               
               
                 2, 20.1 
                 GAAAAAGGAA ATATCTTCCC 
                 [SEQ ID NO:11] 
               
               
                   
                 CT 
               
               
                 3.1 
                 TTTACCCCTT TCTTTTC 
                 [SEQ ID NO:12] 
               
               
                 3.2 
                 GATAGAAAAG GAAA 
                 [SEQ ID NO:13] 
               
               
                 3.3 
                 GGTAGAAAAG GAAA 
                 [SEQ ID NO:14] 
               
               
                 3.4 
                 CTTTCCTTTA GAAAACAGCA 
                 [SEQ ID NO:15] 
               
               
                   
                 GAG 
               
               
                 5.1 
                 GGAAAAkAG GATA 
                 [SEQ ID NO:16] 
               
               
                 5.2 
                 CTTTCTCCTT ACTT 
                 [SEQ ID NO:17] 
               
               
                 6.1 
                 GAAAAGGGAG GTTTCACTCT 
                 [SEQ ID NO:18] 
               
               
                   
                 TT 
               
               
                 6.2 
                 CTTTCTCTAC CAAAAGAAAG 
                 [SEQ ID NO:19] 
               
               
                   
                 G 
               
               
                 7.1 
                 GGTGAAAATG GAAAAGGAAA 
                 [SEQ ID NO:20] 
               
               
                 7.2 
                 GAGGCAAATG GAGAAAAAG 
                 [SEQ ID NO:21] 
               
               
                 7.3 
                 TCCTTTCTTT TCATTC 
                 [SEQ ID NO:22] 
               
               
                 7.4 
                 ACAGAGGAAA AGGAAA 
                 [SEQ ID NO:23] 
               
               
                 7.5 
                 ATGGAGGAAA AGGAAA 
                 [SEQ ID NO:24] 
               
               
                 9.1 
                 AGAGATGAAC CTTTCTTTTT 
                 [SEQ ID NO:25] 
               
               
                 10.1 
                 ACGGGGAGAA AGGAAATA 
                 [SEQ ID NO:26] 
               
               
                 10.2 
                 ATGGAGAGAA AGGAAATA 
                 [SEQ ID NO:27] 
               
               
                 10.3 
                 AGAGGGAGCA GAGGTGAAA 
                 [SEQ ID NO:28] 
               
               
                 10.4 
                 TTTCTCCTTT CTCTTCAT 
                 [SEQ ID NO:29] 
               
               
                 10.5 
                 TTCCACCTTT CTTTTC 
                 [SEQ ID NO:30] 
               
               
                 10.6 
                 CCTTTCTTGA GAGAGAGCAG 
                 [SEQ ID NO:31] 
               
               
                   
                 A 
               
               
                 10.7 
                 TTTCACCTTT CTCTTC 
                 [SEQ ID NO:32] 
               
               
                 10.8 
                 CCTTCCTTTA GAGAGAGCAG 
                 [SEQ ID NO:33] 
               
               
                   
                 A 
               
               
                 10, 12.1 
                 AAAGGTAGAA AAGGAAACA 
                 [SEQ ID NO:34] 
               
               
                 10, 12.2 
                 GAAGAGAAAG GTGAAA 
                 [SEQ ID NO:35] 
               
               
                 10, 12.3 
                 CCTTTCTTTT GATGGAGGAG 
                 [SEQ ID NO:36] 
               
               
                 10, 12.4 
                 CTCTCTCTAA AGAAAGG 
                 [SEQ ID NO:37] 
               
               
                 10, 12.5 
                 AAAGGTAGAA AAGGAAATA 
                 [SEQ ID NO:38] 
               
               
                 12.1 
                 CCTTTCTTTT GATGAAGGAG 
                 [SEQ ID NO:39] 
               
               
                 12.2 
                 GGAAACGGGA TTTCTTCCT 
                 [SEQ ID NO:40] 
               
               
                 12.3 
                 CCTTTCTTTT GATGAAGGAG 
                 [SEQ ID NO:41] 
               
               
                 13, 21.1 
                 GGTGAAAAAG GGAA 
                 [SEQ ID NO:42] 
               
               
                 13, 21.2 
                 GAAAAAGGGA ATGTCTTCCC 
                 [SEQ ID NO:43] 
               
               
                 13, 21.3 
                 AGAGTGGAAC CTCTCTCTTT 
                 [SEQ ID NO:44] 
               
               
                   
                 T 
               
               
                 14, 22.1 
                 GGTGAGAAAG GAAA 
                 [SEQ ID NO:45] 
               
               
                 14, 22.2 
                 AGAGGTGGAT CTTTCTTTT 
                 [SEQ ID NO:46] 
               
               
                 14, 22.3 
                 AAAGGGAATA TCTTCCCCT 
                 [SEQ ID NO.47] 
               
               
                 14, 22.4 
                 GGTGAAAAGG GAAA 
                 [SEQ ID NO:48] 
               
               
                 14, 22.5 
                 GAAAAGGGAA ATATCTTCTC 
                 [SEQ ID NO:49] 
               
               
                 14, 22.6 
                 AACAGAGAAG AACCTTCCTT 
                 [SEQ ID NO:50] 
               
               
                   
                 TT 
               
               
                 16.1 
                 TTTCACCTTT CTTTTC 
                 [SEQ ID NO:51] 
               
               
                 17.1 
                 AGAAAGAAGA CAGAAG 
                 [SEQ ID NO:52] 
               
               
                 17.2 
                 AAAAAGAAGA CAGAAG 
                 [SEQ ID NO:53] 
               
               
                 17.3 
                 TTTTTTTTCC TCTCT 
                 [SEQ ID NO:54] 
               
               
                 17.4 
                 CTTTCCTTTC GAGAGAAG 
                 [SEQ ID NO:55] 
               
               
                 17.5 
                 ITTTCCTTTC GAGAGAGAAG 
                 [SEQ ID NO:56] 
               
               
                 17.6 
                 GGAAAAGGAA TTATCTTTCC 
                 [SEQ ID NO:57] 
               
               
                   
                 C 
               
               
                 18.1 
                 AAGGTGAAAA AGGAAA 
                 [SEQ ID NO:58] 
               
               
                 18.2 
                 ATGGAGAGAA AGGAAA 
                 [SEQ ID NO:59] 
               
               
                 18.3 
                 AAGGTGAAAA AGAAA 
                 [SEQ ID NO:60] 
               
               
                 18.4 
                 AAAGGAGTAG AACCTTTCTT 
                 [SEQ ID NO:61] 
               
               
                   
                 TTC 
               
               
                 20.1 
                 GGTGAAAAAG GAGA 
                 [SEQ ID NO:62] 
               
               
                 20.2 
                 GGTGAAAAAG GAAA 
                 [SEQ ID NO:63] 
               
               
                 20.3 
                 GAAAAAGGAA ATATCTTCCC 
                 [SEQ ID NO:64] 
               
               
                 22.1 
                 GGTGGAGAAG GAAA 
                 [SEQ ID NO:65] 
               
               
                 22.2 
                 GGAAAAGAAT TATCTTCTC 
                 [SEQ ID NO:66] 
               
               
                 22.3 
                 GGTGGAAAAG GAAA 
                 [SEQ ID NO:67] 
               
               
                 22.4 
                 AGTGGAAAAG GAAA 
                 [SEQ ID NO:68] 
               
               
                 22.5 
                 TTCCCTTTCA GAGAGCAG 
                 [SEQ ID NO:69] 
               
               
                 22.6 
                 AAAGGAAATA TCTTCCCCT 
                 [SEQ ID NO:70] 
               
               
                 X.1 
                 ACAGAAAGAC GAGAGAGAAG 
                 [SEQ ID NO:71] 
               
               
                   
                 CA 
               
               
                 X.2 
                 CCTTTTCCTT TATCTTC 
                 [SEQ ID NO:72] 
               
               
                 X.3 
                 GGAAAAGGAA ATATCTTCTC 
                 [SEQ ID NO:73] 
               
               
                   
                 C 
               
               
                 Y.1 
                 GGAAGATGGT GGAAAAGGAA 
                 [SEQ ID NO:74] 
               
               
                   
                 A 
               
               
                 Y.2 
                 GGAAAAGGAA GTATCTTCCT 
                 [SEQ ID NO:75] 
               
               
                   
               
             
          
         
       
     
     TISH is also applicable to other multicopy sequences, as well as to single copy sequence identification, if those sequences are amenable to third-strand binding. In that event, they would have the same utility as FISH for diagnosing genetic disorders, screening for individuals at risk for developing genetic-based diseases, and for diagnosing infectious diseases by detecting the presence of target sequences unique to a pathogen. Complementary diagnostic roles for chromosome-specific α-satellite third-strand probes are as determinants of numerical chromosome abnormalities, i.e., aneuploidy (e.g., trisomy 21), in metaphase and interphase cells of tumors, and of individuals with genetic disorders. TISH-based aneuploid detection is particularly advantageous for those cell types having small nuclear volumes and extremely condensed chromatin, e.g., uncultured prenatal amniocyte fluid cells and spermatozoa, since interphase chromatin denaturation is not required, and third-strand penetration of such nuclei under solution conditions may be more efficient. It is also noted that while FISH and G-banding are incompatible, the former requiring denatured chromosomal DNA and the latter non-denatured DNA, TISH shares that requirement with G-banding, so that those two techniques are applicable to the same mitotic chromosome preparations. In addition, the meiotic stability of chromosome-specific α-satellite polymorphisms (8, 9) should, in cases where third-strand target sequences are heteromorphic, e.g., D17Z1, permit TISH to serve as a rapid assay for Mendelian segregation analysis. 
     Third-strand hybridization of chromosome-specific α-satellite probes to interphase chromatin within nuclei may provide a tool for studying chromosome organization under aqueous conditions. Hybridization of such probes to native metaphase chromosomes within mitotic cells in suspension also makes possible third strand-based chromosome-specific univariate flow sorting. 
     The use and novel features of the present invention will be further understood in view of the following non-limiting examples. 
     EXAMPLE 1 
     Selection of Chromosome 17 α-Satellite Target Sequence 
     GenBank, EMBL, and GB_New databases were searched for human o-satellite sequences using GCG software (10), and a Homo sapiens α-satellite database was constructed from 336 sequence entries containing confirmed nucleotide sequences. This database was then searched to identify same strand and alternate strand (i.e., where the target sequence switches from one duplex strand to the other) homopurine•homopyrimidine runs of defined length and specificity. Mismatches due to inverted base pairs (base pairs with a pyrimidine residue in the predominantly purine-containing target strand) were limited to one per ten residues (2/20, 3/30, etc.); and the position (internal vs. terminal) and relation (adjacent vs. separate) of two or more mismatches was considered when purine-rich•pyrimidine-rich runs exceeded 19 residues. 
     Three same strand and three alternate strand target sequences of varying specificities were identified for chromosome 17. The nucleotide sequence of the chromosome 17 2.7 kb α-satellite higher-order repeat in recombinant clone p17H8 (11) was searched for matches to the six potential target sequences. Identical sequences were found in higher-order repeat monomers 11 and 16 for one alternate and one same strand target sequence, respectively. Similar sequences with mismatches were found for two others, one differing from the monomer 16 sequence by a single point mutation, and no matches were found for the remaining two. The unique 16 bp purine-rich•pyrimidine-rich chromosome 17 α-satellite sequence located in monomer 16, and contained on p17H8, was selected for in vitro and in situ third-strand targeting. 
     Molecular Organization of Chromosome 17 α-Satellite and Target 
     The predominant organization of α-satellite at the D17Z1 locus of chromosome 17 (FIG. 1A) from recombinant clone p17H8 (11), is representative of the general tandem molecular organization of the complement of α-satellites (12). The D17Z1 locus consists of an array of tandem 2.7 kb higher-order repeat units, themselves composed of 16 divergent monomer sequences, each ˜171 bps in length. The 500-1000 tandem repeats separated by unique EcoR1 restriction sites at their junctions, gives rise to arrays of 1-2 million base pairs. Less abundant 15- and 14-monomer repeat units, EcoR1 heteromorphs, are also found constituitively on chromosome 17, with the 15- and 14-mers shown to be essentially identical in sequence to the 16-mer, excluding just 1 and 2 tandem monomers, respectively (11, 13). 
     The chromosome 17 alpha-satellite sequence selected as the target for third-strand binding should be unique, multicopy, and chromosome-specific. The complete nucleotide sequence of the 2.7 kb higher-order repeat on chromosome 17 (11) shows substantial sequence divergence (up to −30%) among its sixteen 171 bp monomers, but not within the 16 bp third-strand target sequence. Independent clones of this tandem 2.7 kb higher-order repeat are &gt;99% identical. Moreover, sequence comparisons among the multimeric alpha-satellite higher-order repeats of human chromosomes show at least 15-30% divergence (12). 
     The purine-rich DNA target strand, 
     AAAAAGAAGA CAGAAG [SEQ ID NO:1] of the selected 16 bp chromosome 17 α-satellite target sequence is interrupted by one pyrimidine (C) residue. Within the megabase chromosome 17 α-satellite array, this 500-1000 times-repeated target sequence occurs once within monomer 16 of each tandem 2.7 kb higher-order repeat (11) (FIG.  1 A). Within simple sequence DNA, the chance occurrence of any 16 bp sequence is 416 or 1 in 4.3×10 9 . Hence, the probability of a non-alpha satellite location for the multicopy chromosome 17 target sequence should be remote in the 3×10 9  base pair haploid human genome. 
     Oligodeoxyribonucleotide Third Strand Probe 
     The 16 nucleotide homopyrimidine third strand DNA probe: 
     TTTTTCTTCT TTCTTC [SEQ ID NO:2] modified at its 5′ and 3′ termini with psoralen and biotin, respectively, binds the 16 bp α-satellite target sequence in parallel orientation to its purine strand complement (FIG.  1 B). m 5 C (5-methylcytosine) residues were substituted for C to afford greater affinity for target G•C base pairs (14). Binding specificity of this third strand to its purine-rich target is derived from the formation of 15 canonical triplets, 11 T:A•T and 4 m 5 C + :G•C. The non-canonical T:C•G triplet sandwiched between T:A•T nearest neighbors is only moderately destabilizing on similar Y:R•Y triplexes (4). 
     The [SEQ ID NO:2] third strand and its dual 5′-psoralen- and 3′-biotin-modified version were synthesized by the β-cyanoethyl phosphoramidite method with an Applied Biosystems Synthesizer 380B. A psoralen (4′-hydroxymethyl-4,5′,8-trimethylpsoralen, HMT) C6 phosphoramidite and biotin TEG-CPG column (both Glen Research) were used to incorporate psoralen and biotin at respective 5′ and 3′ termini. The annotation 5′-Pso-Tm 5 C-Bio-3′ represents dual psoralen- and biotin-modified third strands, and 5′-Tm 5 C-3′, the non-modified form. 
     Crude synthesis products were analyzed by PAGE and UV shadowing. Oligomers were gel purified, desalted using Water&#39;s Sep Pak C18 cartridges, lyophilized, and stored at −20° C. in sterile water. The concentration of oligonucleotides was determined in ddH 2 O at A 260  using extinction coefficients of 8800 and 5700 M −1  cm −1  for dT and dm 5 C residues, respectively. 
     Cocktail for Third-Strand In Vitro and In Situ Hybridization 
     Both quantitative solution and in situ third-strand hybridization assays were carried out in 25 μl of 10 mM Bis-Tris HCl-buffered cocktail at pH 6.0 containing optimal concentrations of mono-, di-, and polyvalent cations (50 mM K + /10 MM Mg 2+ /1 μM spermine 4+ ), reducing and chelating agents (1 mM DTT and 1 mM EDTA), a molecular crowding agent (2% PEG 8000), deionized formamide (1.0-2.5%), and alcohol-precipitated (100-300 nt) total yeast RNA (1.0 ng). For in situ hybridization, the chaotropic agent formamide was used at low concentrations (≦2.5%) to reduce the capillary adhesion between glass microscope slide and coverslip surfaces, so as to promote third-strand equilibration across the total hybridizable surface area. Formamide at these concentrations lowers the Tm of duplex DNA by only 0.7-1.75° C. Yeast RNA was used to compete out non-specific binding of third strands to glass surfaces and cellular debris. Neither deionized formamide nor yeast RNA at these concentrations affect the specificity or affinity of third-strand binding to the p17H8 chromosome 17 target sequence in solution. 
     Third-Strand In Vitro Hybridization and Binding Specificity 
     Optimized solution binding assays confirmed third-strand binding specificity to chromosome 17 α-satellite target sites in various sized recombinant DNAs (5.7 to 50 kb) (15). One such assay is shown to demonstrate third-strand binding specificity to this target in a supercoiled plasmid; another, to confirm that this specificity is not diminished in the presence of a human chromatin background similar in DNA sequence complexity to a fixed protein-depleted mitotic spread. 
     Thus, incubation of radiolabeled third-strand probe lacking psoralen and biotin moieties, 5′-Tm 5 C-3′, with different supercoiled plasmid substrates demonstrated that third-strand binding is specific for the plasmid carrying the 16 bp α-satellite target sequence. Plasmid substrates were p17H8 (5.7 kb), which contains the complete 2.7 kb chromosome 17 α-satellite higher-order repeat, and pMJ1 (˜5.5 kb), a p17H8 deletion-derivative lacking the target sequence and an additional 158 bp of monomer 16 but containing monomers 1-15. Supercoiled p17H8 and pMJ1 were mixed with  32 P-end-labeled third strand at an oligomer:plasmid molar ratio of 1:1 in 1×triplex cocktail±formamide (0.25 or 2.5%) and yeast RNA (1.0 ng) at pH 6.0 and 23° C. for ˜20 hrs to attain equilibrium. FIG. 2A shows  32 P-label at gel positions corresponding to supercoiled (sc) and open circular (oc) p17H8 (lanes 2-4). The deletion plasmid pMJ1, which lacks the target sequence but contains monomers 1-15, was not bound by the labeled third strand (lanes 5-7). 
     The degree of sequence similarity between each of the 16-171 bp monomers that make up the α-satellite higher-order repeat of chromosome 17 provides an “internal control” for third-strand binding specificity. Sequences in monomers 1-15 are similar in length and base composition to the monomer 16 target sequence, one differing from it by only two bases (11). Hence, this solution binding assay shows that, under the conditions employed, third-strand binding is highly specific for the 16 bp sequence within monomer 16 of the 2.7 kb chromosome 17 α-satellite repeat on p17H8. 
     The specificity of third-strand binding to p17H8 was also examined in the presence of increasing amounts of unsorted human metaphase chromosomes (1-22, XY). In these solution binding assays, the stoichiometry of  32 P-end-labeled third strand, 5′-Tm 5 C-3′, to p17H8 was 2:1. This permitted any non-specific binding to chromatin DNA, histone and non-histone proteins, or RNA to be detected as decreased plasmid-bound  32 P-third strand. The amount of alpha-satellite target sequence due to chromosome 17 (˜1:24) was insufficient to compete against plasmid targets. After 1 hr incubations, plasmid signal intensity was observed to decrease slightly as unsorted chromosomes increased from ˜24 to ˜2,400. FIG. 2B shows the results of a kinetic experiment that examined this rate-limiting effect on binding to plasmid in the presence of ˜2,400 unsorted metaphase chromosomes. It can be seen that sc and oc plasmid-bound  32 P-third-strand signals increase over time, t o =1 hr. Thus, ˜2,400 chromosomes, i.e., 6×10 11  bps of unsorted human chromosomal DNA, did not detectably inhibit or impair the specificity of third-strand binding to p17H8. It merely slowed the kinetics. This result suggested that specificity was achievable for third-strand binding to unique and accessible alpha-satellite target sequences on chromosome 17 in mitotic spreads and nuclei. 
     Protocol for Third-Strand In Situ Hybridization (TISH) 
     TISH was performed at pH 6.0 under non-denaturing solution conditions that give quantitative third-strand binding to recombinant DNA molecules. A general protocol is as follows: 
     Solution Conditions: 1×triplex cocktail for TISH contains 10 mM Bis-Tris-HCl/50 mM KCl/10 mM MgCl 2 /1 mM DTT/1 mM EDTA/1 μM spermine/2% PEG 8000, pH 6.0 at 23° C. TISH pre-hybridization, post-hybridization, and UVA irradiation buffers are slight variations of this cocktail. The pH of Bis-Tris buffers is temperature sensitive, ΔpH/Δt=−0.014 (pH units/°C.). At the elevated temperatures used for hybridization (45° C.) and post-hybridization washing (51° C.), the pH of TISH buffers is 5.7 and 5.6, respectively. 
     Pre-hybridization: Room temperature slides containing metaphase spreads and interphase nuclei were dehydrated in successive 2 minute immersions in 70, 80, and 95% ethanol (at 23° C.) incubations. After air-drying, the slides were incubated in TISH pre-hybridization buffer [10 mM Bis-Tris-HCl/50 mM KCl/2.5 mM EDTA/1 mM DTT/pH 6.0] at room temperature for 30 minutes to promote chromosome swelling. For all remaining steps, the slides were never allowed to dry. 
     Hybridization: TISH hybridization mixtures (final vol. 25 μl) contain 1×triplex cocktail (pH 6.0) supplemented with 1.0-2.5% deionized formamide (BRL, ultrapure), 1.0 ng of blocking RNA (alcohol precipitated, size fractionated total yeast RNA), and ˜20 ng of third-strand probe. Hybridization mixtures and slides were pre-heated at 45° C. for 15 minutes in a water bath and humidified chamber, respectively, after which the mixtures were applied to the slides under 22 mm 2  glass coverslips. Hybridization was performed in the pre-warmed humidified chamber for 2 hours at 45° C. 
     Post-Hybridization Washing: Slides with coverslips intact were immersed into TISH washing buffer [10 mM Bis-Tris-HCl/75 mM KCl/10 mM MgCl 2 /1 mM DTT/pH 5.6] at 51° C., agitated to loosen the coverslips, and incubated further for 5 min. 
     UVA Photochemistry: After removal from the wash buffer, slides were quickly covered with 22 mm 2  coverslips to which 25 μl of DTT/EDTA-depleted 1×cocktail, pH 6.0, was applied. The slides were then placed in an open petri dish and UVA (320-400 nm) irradiated for 5 minutes (14 watt low pressure mercury arc, model #RPR 3500 Å, The Southern New England Ultraviolet Company, Branford, Conn.). Typical UV irradiance was 1.6 J/cm 2  per 5 minutes at 320-400 nm. Afterwards, the slides with coverslips intact were placed in 1×PBD, pH 8.0 (phosphate buffer with 0.05% Nonidet P40) at room temperature and agitated to loosen the coverslips. The incubation time in 1×PBD is not critical. Incubation times were generally 2-5 minutes, the longest 1 hour. 
     Cytochemical Detection: Slides were removed from the 1×PBD, and 60 μl of fluorescein (FITC)-labeled avidin (Oncor, Inc., Gaithersburg, Md.) was applied under a plastic coverslip. After incubation in a pre-warmed humidified chamber at 37° C. for 20 minutes, their plastic coverslips were carefully removed, and the slides then washed three times for 2 minutes each in 1×PBD, pH 8.0, at room temperature. After the final wash, excess fluid was blotted from the slide edges, and 15 μl of propidium iodide (PI)/antifade, pH 8.5, (final PI conc. 0.3 mg/ml) (Oncor, Inc.) was applied under a 22 mm 2  coverslip. The slides were then incubated for 5 minutes at room temperature. 
     Epi-fluorescent Microscopy: Fluorescent signals were captured and stored using an Intel pentium powered Oncor Archive 2.0 color imaging system (Oncor, Inc.) consisting of a Zeiss Epi-fluorescent microscope equipped with a 3CCD high resolution color camera controlled by a digital image processor. 
     Hydrated slides containing non-denatured metaphase spreads and interphase nuclei, isolated by standard cytogenetic methods from human lymphocytes, and a hybrid mouse×human chr. 17 fibroblast cell line (cell line GM10498, Coriell Institute for Medical Research, Camden, N.J.), were incubated with 25 μl of 1×triplex hybridization mixtures containing 20 ng of chromosome 17 α-satellite-specific third strand probe, 5′-Pso-Tm 5 C-Bio-3′. Optimal parameters for TISH: pre-hybridization hydration (30 minutes at 23° C.), hybridization (2 hours at 45° C.), post-hybridization washing (5 minutes at 51° C.), and UVA (320-400 nm) irradiation (5 minutes) were determined empirically. Post-hybridization washing at elevated temperature and volume effectively promotes dissociation of dual psoralen- and biotin-modified third strands bound to non-target duplex sequences. Subsequent UVA-induced psoralen-DNA photoadduct formation prevents dissociation of target-specific bound third strands containing m 5 C +  residues at the alkaline pH conditions required for pre-detection wash and FITC-avidin detection. Chromosome 17 centromere-specific fluorescence hybridizations with high signal to noise ratios were obtained in the absence of RNAse A and protease treatment. Moreover, no signal amplification was required. 
     Two approaches were taken to establish that the DNA of non-denatured fixed metaphase spreads and interphase nuclei prepared for TISH are in a duplex state. One exploits fluorescent in situ hybridization (FISH), which requires single-stranded targets. FISH assays performed on non-denatured spreads and nuclei using a standard chromosome 17 D17Z1-specific ˜200 bp biotin-labeled probe produced neither centromere-specific nor non-specific fluorescent signals (FIG.  3 ). The other is based on the general chromosome staining intercalator propidium iodide (PI), which has higher affinity and exhibits greater fluorescence when bound to double-stranded (ds) than to single-stranded (ss) DNA (16). Counterstaining of TISH-assayed non-denatured spreads and nuclei was performed at PI concentrations used for FISH, in which case renaturation in 70% formamide is not quantitative, and should yield a mixture of ds and ss DNA. Non-denatured TISH spreads emitted significantly brighter PI fluorescence than renatured FISH spreads. Together, these observations confirm a duplex state for both chromosome 17 α-satellite and the totality of mitotic chromosomal DNA on slides prepared for TISH. 
     Chromosome 17-Specific Identification by TISH 
     A typical in situ hybridization of 5′-Pso-Tm 5 C-Bio-3′ to a non-denatured fixed metaphase spread and nuclei from a karyotypically normal 46, XY male lymphocyte cell line is shown in FIGS. 4A and B. Detection did not require amplification. The two distinct centromere-specific yellow-green fluorescent spots identifying homologous chromosomes 17 in the metaphase spread and three interphase nuclei were reproduced in ˜90% of all spreads and nuclei examined. The other 10% emitted short-lived fluorescent signals that were not capturable due to quenching. More than 250 combined spreads and nuclei per individual examined (5 subjects in total) were scored to ensure statistical significance. Metaphase and prometaphase (not shown) chromosomes 17 were equally labeled. The spatial organization of the fluorescent signals in the three male subject nuclei, one peripherally located, the other more central, was consistent for the bulk of nuclei examined. The spreads and nuclei in FIGS. 4A and B show the empirically determined fluorescence maximum observed after a 2 hour hybridization at 45° C. with 20 ng of probe. Longer hybridizations with 20 ng or more probe did not elicit stronger fluorescent signals, while less time and/or probe produced weaker signals. Under TISH hybridization conditions, saturation of all accessible chromosome 17 α-satellite targets was reproducibly achieved within 2 hours. 
     The specificity of third-strand probe binding to in situ mitotic chromosomes 17 was additionally confirmed. One approach involved mouse×human somatic cell hybrid metaphase spreads and nuclei containing a single human chromosome 17. Another involved TISH competition assays, in which increasing concentrations of non-covalently modified third strand, 5′-Tm 5 C-3′, were added to compete against 5′-Pso-Tm 5 C-Bio-3′-specific binding. FIGS. 5A and B show the centromere-specific third strand-based fluorescence of single human metaphase and interphase chromosome 17s in a mouse genomic background. A competition assay (FIG. 5C) demonstrates that when 5′-Pso-Tm 5 C-Bio-3′ is competed out, there is no non-specific binding to the remaining human mitotic chromosomes in spreads. 
     α-Satellite Target Sequence Polymorphism Detected by TISH 
     Third-strand in situ hybridizations were performed on non-denatured metaphase spreads and nuclei of 5 unrelated individuals, 4 male and 1 female. Three centromere-specific chromosome 17 fluorescent signal patterns were recorded: (+17/+17 homozygotes) when each chromosome 17 homologue was labeled, (+17/−17 heterozygotes) when only one homologue was labeled, and (−17/−17 homozygotes) when neither emitted detectable fluorescence. In contrast, all five individuals yielded detectable +17/+17 FISH-based signals with a ˜200 bp biotin-labeled D17Z1 α-satellite probe. FIGS. 6A-D show contrasting TISH and FISH images for two signal patterns. The two variant forms of chromosome 17 (+17 and −17) identified by TISH represent at least two distinct D17Z1 haplotypes. 
     Inter- and intra-homologue sequence variations identified within the D17Z1 loci of chromosomes 17 can alter effective third-strand target copy number, and so could account for the two variant D17Z1 haplotypes. Inter-homologue sequence variations within α-satellite higher-order repeats of paired chromosomes 17 are characterized by single point mutations in either simple or restriction-enzyme sequences (RFLPs), and as differences in repeat length due to deletions of single or multiple contiguous ˜171 bp monomer units (8, 17-19). Data from Warburton and Willard (20) suggest that a partial explanation for the observed variations in third-strand binding to different D17Z1 loci could be target sequence heterogeneity due to a single A→G transition mutation of the second residue (counting 5′→3′) of the purine-rich target strand. 5′-Pso-Tm 5 C-Bio-3′ third-strand binding affinity would likely be significantly impaired by this mutation at the TISH hybridization temperature of 45° C. since a T:G•C mismatch replaces a canonical T:A•T triplet. In fact, a single T:G•C mismatch in a similar Y:R•Y nearest neighbor environment prohibited third-strand binding at ambient and elevated temperatures (4). 
     Intra-homologue sequence variation within the megabase ∝-satellite array of individual chr. 17s (18) is characterized by highly amplified localized homogeneous domains containing one distinct type of higher-order repeat unit differing in either sequence or length from flanking repeat units. The existence within different chr. 17 ∝-satellite arrays of such domains containing the altered target sequence (A→G transition) flanked by repeats containing wild-type target sequences could, depending on their respective ratios, further account for the observed variability in third-strand binding. D17Z1 loci having many fewer wild-type target sequences might be expected to exhibit reduced TISH-based fluorescent signals or even none at all at 45° C. In this respect, TISH affords a sensitivity not inherent in FISH. 
     Third-Strand Target Accessibility 
     Third-strand probe binding to duplex targets in solution can be described by the following equilibrium:        Ts   +     D          ←   →       k     1      off         k     2      on            Ts                 D                            
     where Ts is the third strand, D is the duplex target, TsD the triplex, and k2on and k1off second-order association and first-order dissociation rate constants. When third strand concentration is in excess to a duplex target, and binding is strong, the reaction is essentially irreversible, and like Watson-Crick reassociation kinetics, pseudo-first-order. Such is the case for third strand titrations to a duplex target of equal length, and to single targets contained within relatively large DNA fragments (≦400 base pairs) or supercoiled plasmids (3). For reactions involving such molecules, the equilibrium lies far to the right. 
     The TISH experiments described here demonstrate that a 16 residue third strand can bind with specificity to a unique multicopy centromeric target sequence on metaphase and interphase chr. 17s. This result suggests that solvent-exposed areas of non-denatured fixed mitotic chromosomes on slides are accessible to and explorable by third strands. The reproducibility of chr. 17 centromere-specific fluorescent labeling with high signal to noise ratios confirms that third strands can associate with specific chromosomal sequences and dissociate from non-specific ones on slides under solution conditions. This accessibility suggests that in such spreads there are no significant barriers to third strands directed to non-∝-satellite target sequences. 
     References 
     1. G. Felsenfeld and A. Rich,  Biochim Biophys Acta  26, 457 (1957); J. R. Fresco and B. M. Alberts,  Proc. Natl. Acad. Sci. USA . 46, 311 (1960); J. R. Fresco, in  Informational Macromolecules , H. J. Vogel, V. Bryson, and J. O. Lampen, Eds. (Academic Press, N.Y., 1963), pp. 121-142; H. E. Moser and P. B. Dervan,  Science  238, 645 (1987); T. Le Doan, L. Perrouault, D. Praseuth, N. Habhoub, J. L. Decout, N. T. Thoung, J. Lhomme, C. Hélène,  Nucleic Acids Res . 15, 7749 (1987). 
     2. A. G. Letai, M. A. Palladino, E. Fromm, V. Rizzo, J. R. Fresco,  Biochemistry  27, 9108 (1988). 
     3. L. J. Maher, P. B. Dervan, B. Wold,  Biochemistry  29, 8820 (1990); P. W. Roberts and D. M. Crothers,  Proc. Natl. Acad. Sci. USA . 88, 9397 (1991); S. F. Singleton and P. B. Dervan,  Biochemistry  31, 10995 (1992); S. F. Singleton and P. B. Dervan,  J Am Chem Soc , 114, 6957 (1992); M. Rougee, B. Faucon, J. L. Mergny, F. Barcelo, C. Giovannangeli, T. Garestier, C. Hélène,  Biochemistry  31, 9269 (1992). 
     4. J. A. Fossella, Y. J. Kim, H. Shih, E. G. Richards, J. R. Fresco,  Nucleic Acids Res . 21, 4511 (1993); K. Yoon, C. A. Hobbs, J. Koch, M. Sardaro, R. Kutny, A. L. Weis,  Proc. Natl. Acad. Sci. USA . 89, 3840 (1992); G. C. Best and P. B. Dervan,  J Am Chem Soc  117, 1187 (1995). 
     5. E. H. Postel, S. J. Flint, D. J. Kessler, M. E,. Hogan,  Proc. Natl. Acad. Sci. USA . 88, 8227 (1991); W. M. McShan, R. D. Rossen, A. H. Laughter, J. Trial, D. J. Kessler, J. G. Zendegui, M. E. Hogan, F. M. Orson,  J. Biol. Chem . 267, 5712 (1992); C. Giovannangeli, S. Diviacco, V. Labrousse, S. Gryaznov, P. Charneau, C. Hélène,  Proc. Natl. Acad. Sci. USA . 94, 79 (1997). 
     6. S. A. Strobel, L. A. Doucette-Stamm, L. Riba, D. E. Housman, P. B. Dervan,  Science  254, 1639 (1991). 
     7. D. Pinkel, T. Straume, J. W. Gray,  Proc. Natl. Acad. Sci. USA . 83, 2934 (1986); B. J. Trask,  Methods Cell Biol . 35, 3 (1991). 
     8. R. Wevrick and H. F. Willard,  Proc. Natl. Acad. Sci. USA . 86, 9394 (1989). 
     9. H. F. Willard, G. M. Greig, V. E. Powers, J. S. Waye,  Genomics . 1, 368 (1987). 
     10. The Genetics Computer Group (GCG) software from the University of Wisconsin Genetics Computer Group, Inc. 
     11. J. S. Waye and H. F. Willard,  Mol. and Cell. Biol . 6, 3156 (1986). 
     12. H. S. Willard and J. S. Waye,  Trends Genet . 3, 192 (1987). 
     13. J. S. Waye and H. F. Willard,  Nucleic Acids Res . 14, 6915 (1986). 
     14. T. J. Povsic and P. B. Dervan, J. Am. Chem. Soc. 111, 3059 (1989); G. E. Plum, Y. W. Park, S. F. Singleton, P. B. Dervan, K. J. Breslauer,  Proc. Natl. Acad. Sci. USA . 87, 9236 (1990); L. E. Xodo, G. Manzini, F. Quadrifoglio, G. A. van der Marel, J. H. van Boom,  Nucleic Acids Res . 19, 5625 (1991). 
     15. M. D. Johnson and J. R. Fresco, in preparation. 
     16. E. J. Gabbay and W. D. Wilson,  Methods Cell Biol . 18, 351 (1978). 
     17. H. F. Willard, J. S. Waye, M. H. Skolnick, C. E. Schwartz, V. E. Powers, S. B. England,  Proc. Natl. Acad. Sci. USA . 83, 5611 (1986). 
     18. P. E. Warburton and H. F. Willard,  J. Mol. Biol . 216, 3 (1990). 
     19. P. E. Warburton and H. F. Willard,  J. Mol. Evol . 41, 1006 (1995). 
     20. P. E. Warburton and H. F. Willard,  Nucleic Acids Res.  20, 6033 (1992). 
     
       
         
           
             77 
           
           
             1 
             16 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  purine
      rich DNA target strand 
             
           
            1
aaaaagaaga cagaag                                                     16
 
           
             2 
             16 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence
      homopyrimidine third strand DNA probe 
             
           
            2
tttttcttct ttcttc                                                     16
 
           
             3 
             14 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            3
agtaaaggaa agaa                                                       14
 
           
             4 
             18 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            4
tatttccttt tctcgctt                                                   18
 
           
             5 
             15 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            5
gaatgaaaag gaaag                                                      15
 
           
             6 
             17 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            6
tttccttttc tcgcttt                                                    17
 
           
             7 
             19 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            7
aaaggtagaa aaggaaata                                                  19
 
           
             8 
             19 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            8
aaaggcagaa aaggaaata                                                  19
 
           
             9 
             16 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            9
cttttctttt tcattc                                                     16
 
           
             10 
             20 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            10
cctttctttt gagagagcag                                                 20
 
           
             11 
             22 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            11
gaaaaaggaa atatcttccc ct                                              22
 
           
             12 
             17 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            12
tttacccctt tcttttc                                                    17
 
           
             13 
             14 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            13
gatagaaaag gaaa                                                       14
 
           
             14 
             14 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            14
ggtagaaaag gaaa                                                       14
 
           
             15 
             23 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            15
ctttccttta gaaaacagca gag                                             23
 
           
             16 
             14 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            16
ggaaaaaaag gata                                                       14
 
           
             17 
             14 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            17
ctttctcctt actt                                                       14
 
           
             18 
             22 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            18
gaaaagggag gtttcactct tt                                              22
 
           
             19 
             21 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            19
ctttctctac caaaagaaag g                                               21
 
           
             20 
             20 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            20
ggtgaaaatg gaaaaggaaa                                                 20
 
           
             21 
             19 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            21
gaggcaaatg gagaaaaag                                                  19
 
           
             22 
             16 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            22
tcctttcttt tcattc                                                     16
 
           
             23 
             16 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            23
acagaggaaa aggaaa                                                     16
 
           
             24 
             16 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            24
atggaggaaa aggaaa                                                     16
 
           
             25 
             20 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            25
agagatgaac ctttcttttt                                                 20
 
           
             26 
             18 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            26
acggggagaa aggaaata                                                   18
 
           
             27 
             18 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            27
atggagagaa aggaaata                                                   18
 
           
             28 
             19 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            28
agagggagca gaggtgaaa                                                  19
 
           
             29 
             18 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            29
tttctccttt ctcttcat                                                   18
 
           
             30 
             16 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            30
ttccaccttt cttttc                                                     16
 
           
             31 
             21 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            31
cctttcttga gagagagcag a                                               21
 
           
             32 
             16 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            32
tttcaccttt ctcttc                                                     16
 
           
             33 
             21 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            33
ccttccttta gagagagcag a                                               21
 
           
             34 
             19 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            34
aaaggtagaa aaggaaaca                                                  19
 
           
             35 
             16 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            35
gaagagaaag gtgaaa                                                     16
 
           
             36 
             20 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            36
cctttctttt gatggaggag                                                 20
 
           
             37 
             17 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            37
ctctctctaa agaaagg                                                    17
 
           
             38 
             19 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            38
aaaggtagaa aaggaaata                                                  19
 
           
             39 
             20 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            39
cctttctttt gatgaaggag                                                 20
 
           
             40 
             19 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            40
ggaaacggga tttcttcct                                                  19
 
           
             41 
             20 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            41
cctttctttt gatgaaggag                                                 20
 
           
             42 
             14 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            42
ggtgaaaaag ggaa                                                       14
 
           
             43 
             20 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            43
gaaaaaggga atgtcttccc                                                 20
 
           
             44 
             21 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            44
agagtggaac ctctctcttt t                                               21
 
           
             45 
             14 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            45
ggtgagaaag gaaa                                                       14
 
           
             46 
             19 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            46
agaggtggat ctttctttt                                                  19
 
           
             47 
             19 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            47
aaagggaata tcttcccct                                                  19
 
           
             48 
             14 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            48
ggtgaaaagg gaaa                                                       14
 
           
             49 
             20 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            49
gaaaagggaa atatcttctc                                                 20
 
           
             50 
             22 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            50
aacagagaag aaccttcctt tt                                              22
 
           
             51 
             16 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            51
tttcaccttt cttttc                                                     16
 
           
             52 
             16 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            52
agaaagaaga cagaag                                                     16
 
           
             53 
             16 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            53
aaaaagaaga cagaag                                                     16
 
           
             54 
             15 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            54
ttttttttcc tctct                                                      15
 
           
             55 
             18 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            55
ctttcctttc gagagaag                                                   18
 
           
             56 
             20 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            56
ttttcctttc gagagagaag                                                 20
 
           
             57 
             21 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            57
ggaaaaggaa ttatctttcc c                                               21
 
           
             58 
             16 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            58
aaggtgaaaa aggaaa                                                     16
 
           
             59 
             16 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            59
atggagagaa aggaaa                                                     16
 
           
             60 
             15 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            60
aaggtgaaaa agaaa                                                      15
 
           
             61 
             23 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            61
aaaggagtag aacctttctt ttc                                             23
 
           
             62 
             14 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            62
ggtgaaaaag gaga                                                       14
 
           
             63 
             14 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            63
ggtgaaaaag gaaa                                                       14
 
           
             64 
             20 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            64
gaaaaaggaa atatcttccc                                                 20
 
           
             65 
             14 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            65
ggtggagaag gaaa                                                       14
 
           
             66 
             19 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            66
ggaaaagaat tatcttctc                                                  19
 
           
             67 
             14 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            67
ggtggaaaag gaaa                                                       14
 
           
             68 
             14 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            68
agtggaaaag gaaa                                                       14
 
           
             69 
             18 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            69
ttccctttca gagagcag                                                   18
 
           
             70 
             19 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            70
aaaggaaata tcttcccct                                                  19
 
           
             71 
             22 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            71
acagaaagac gagagagaag ca                                              22
 
           
             72 
             17 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            72
ccttttcctt tatcttc                                                    17
 
           
             73 
             21 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            73
ggaaaaggaa atatcttctc c                                               21
 
           
             74 
             21 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            74
ggaagatggt ggaaaaggaa a                                               21
 
           
             75 
             20 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            75
ggaaaaggaa gtatcttcct                                                 20
 
           
             76 
             20 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  Target
      sequences 
             
           
            76
ctataaaaag aagacagaag                                                 20
 
           
             77 
             20 
             DNA 
             Artificial Sequence 
             
               Description of Artificial Sequence  target
      sequence 
             
           
            77
gatatttttc ttctgtcttc                                                 20