The involvement of specific chromosomes in the formation of micronuclei (MN) is neither well studied nor understood. We studied the involvement of chromosomes 7, 18 and X in mitomycin C (MMC)-induced MN. A total of 1,202 female MN were studied by three-color fluorescence in situ hybridization (FISH) using centromere-specific probes. After a second round of hybridization, using whole chromosome painting probes, 916 MN were evaluated. The frequency of MN containing whole chromosome painting signals for chromosomes X in 916 male MN was analyzed. Different frequency of involvement of selected chromosomes in MN formation was revealed. Chromosome interphase position, size and inactive (heterochroma­tinized) state of X chromosome in females was discussed as possible reasons of their non random involvement in MN formation. There was no clear relationship between chromosome interphase position and size and migration in MN. However, it was shown that X-chromosomal material was over represented in female- and under represented in male-derived MN. We speculated about a preferred inclusion of the inactive female X-chromosome into MN.

      Keywords: Chromosomes 7, 18, X; Fluorescence in situ hybridization (FISH); Mitomycin C (MMC); Micro­nuclei (MN)