Sporadic, non-modal chromosomal spreads may be either hypomodal, in which the number of chromosomes is less than the modal number, or hypermodal, with more than the mode. Hypermodals occur less frequently than hypomodals in routine lymphocytic preparations. The hypomodal spread has generally been considered to be a technical artefact resulting from random loss during the preparation of slides by the splash technique. In contrast, the less frequently occurring hypermodal spread may originate differently and be meaningful. Previously, we called attention to the possibility of their non-random occurrence among the subJects studied in a clinical cytogenetic laboratory.'We showed that the frequency of sporadic, hypermodal spreads from the parents of aneuploid offspring significantly exceeded that found in comparison populations. We suggested mitotic nondisjunction as the origin of these spreads after eliminating other possibilities. Predisposition to lymphocytic mitotic non-disjunction in vitro might reflect an inherent tendency towards nondisjunction, including meiotic non-disjunction which leads to an aneuploid conceptus. An aneuploid conceptus will be either liveborn, with considerable probability of physical and mental abnormalities, or spontaneously aborted. The outcome is particularly dependent on the type of aneuploidy.Here we show that the frequency of sporadic, hypermodal spreads observed in recurrent spontaneous aborters significantly exceeded the frequency in suitable comparison groups. The fre-

Mithramycin inhibits transcription by binding to G/C-rich sequences, thereby preventing regulatory protein binding. However, it is also possible that mithramycin inhibits gene expression by preventing intramolecular triplex DNA assembly. We tested this hypothesis using the DNA triplex adopted by the murine c-myb proto-oncogene. The 5'-regulatory region of c-myb contains two polypurine:polypyrimidine tracts with imperfect mirror symmetry, which are highly conserved in the murine and human c-myb sequences. The DNA binding drugs mithramycin and distamycin bind to one of these regions as determined by DNase I protection assay. Gel mobility shift assays, nuclease and chemical hypersensitivity and 2D-gel topological analyses as well as triplex-specific antibody binding studies confirmed the formation of purine*purine:pyrimidine inter- and pyrimidine*purine:pyrimidine intra-molecular triplex structures in this sequence. Mithramycin binding within the triplex target site displaces the major groove-bound oligonucleotide, and also abrogates the supercoil-dependent H-DNA formation, whereas distamycin binding had no such effects. Molecular modeling studies further support these observations. Triplex-specific antibody staining of cells pretreated with mithramycin demonstrates a reversal of chromosomal triplex structures compared to the non-treated and distamycin-treated cells. These observations suggest that DNA minor groove-binding drugs interfere with gene expression by precluding intramolecular triplex formation, as well as by physically preventing regulatory protein binding.