From an acute B-cell leukemia cell line, a DNA probe was obtained that was specific for chromosome 18 and flanked the heavy chain joining region of the immunoglobulin heavy chain locus on chromosome 14. This probe detected rearrangement of the homologous DNA segment in the leukemic cells and in follicular lymphoma cells with the t(14:18) chromosome translocation but not in other neoplastic or normal B or T cells. The probe appears to identify bcl-2, a gene locus on chromosome 18 (band q21) that is unrelated to known oncogenes and may be important in the pathogenesis of B-cell neoplasms with this translocation.

Man, gorilla, and chimpanzee likely shared an ancestor in whom the fine genetic organization of chromosomes was similar to that of present man. A comparative analysis of high-resolution chromosomes from orangutan, gorilla, chimpanzee, and man suggests that 18 or 23 pairs of chromosomes of modern man are virtually identical to those of our "common hominoid ancestor", with the remaining pairs slightly different. From this lineage, gorilla separated fist, and three major chromosomal rearrangements presumably occurred in a progenitor of chimpanzee and man before the final divergence of these tow species. A precursor of the hominoid ancestor and orangutan is also assumed.

Breaks were observed at 51 sites in homologous chromosomes in lymphocytes from ten humans and two great apes when cells were deprived of thymidine. The incidence of breaks was enhanced by caffeine, a substance that inhibits DNA repair in replicating cells. The locations of 20 sites were correlated with breakpoints that have been related to human malignancy.

High-resolution banding techniques for the study of human chromosomes have revealed that the malignant cells of most tumors analyzed have characteristic chromosomal defects. Translocations of the same chromosome segments with precise breakpoints occur in many leukemias and lymphomas, and a specific chromosome band is deleted in several carcinomas. Trisomy, or the occurrence of a particular chromosome in triplicate, is the only abnormality observed in a few neoplasias. It is proposed that chromosomal rearrangements play a central role in human neoplasia and may exert their effects through related genomic mechanisms. Thus, a translocation could serve to place an oncogene next to an activating DNA sequence, a deletion to eliminate an oncogene repressor, and trisomy to carry extra gene dosage.

G-band prophase chromosomes from synchronized cells permit a high degree of resolution not previously attained in the study of chromosome structure and birth defects in man.