Fig. 2. Distribution of MMC-induced chromatid interchanges over the genome. Frequencies of chromatid interchanges observed in human lymphocytes after treatment in G₁ with 4 µM MMC for 1 hour. (A) The genomic frequency (estimated by analysis of 200 Giemsa-stained metaphase cells) as well as the specific frequencies for chromosomes 1, 2, 8 and 9 (analysis of 900 cells after FISH painting) are plotted. Chisquare statistics showed a highly significant (P<0.001) difference among the four chromosomes used taking into account their DNA contents. No chromatid interchanges were observed in 900 mock-treated cells. (B) The frequencies of chromatid interchanges observed for chromosome 1 and 9 are subdivided into two categories–namely, those between the painted homologues (1-1 or 9-9) and between two non-homologous chromosomes (1-other or 9-other).