The occurrence of chromosome gaps, breaks and rearrangements was determined in 3,720 lymphocyte metaphases obtained from ten normal adults. The frequency of the cells with one or more breaks varied from 1–20%, and the maximum variation found in one individual during sequential cultures was 6–20%. Multiple breaks were found in only 17 cells, and a structural rearrangement (translocation, dicentric inversion or triradial configuration) occurred in eight cells. Chromatid breaks were not randomly distributed, occurring most frequently as subterminal breaks in the long arm of No. 16 in both sexes and the short arm of the No. 3 in females. Previous studies demonstrating an increased number of breaks in these areas following treatment with a variety of agents may be best explained by an accentuation of the normally occurring pattern of bre
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