Nonrandom distribution of chromosome breaks in cultured lymphocytes of normal subjects.

Breakpoint distribution was studied from cultured lymphocytes on 7653 metaphases from 524 subjects whose karyotypes were normal. The mean break rate was 5% in both sexes. The frequency increased significantly after 40 years and varied during the year. The location of the breaks was very different from the expected random distribution. The break frequency for each chromosome was different according to the type of break (chromatid, simple chromosomal and chromosomal involving rearrangements). The location of the breaks was also studied according to type of band and with respect to the centromere. A comparison between spontaneous breaks, X-ray induced breaks, breaks in Fanconi's anemia had in congenital rearrangements, show very significant differences.