Micronucleus formed during mitotic cell division and present in the cytoplasm of daughter cells near the nucleus. The main mechanisms contributing to their creation are chromosome breakage and dysfunction of the mitotic apparatus. Micronuclei may contain whole chromosomes or chromatids, acentric fragments and chromatid fragments. Observed changes in the incidence of micronuclei and content depending on the disease, the aging process, and different environmental factors. The analysis of the number and content of micronuclei is used to assess the degree of chromosome damage at the single cell level and for determining susceptibility to degenerative diseases and cancer, and determining the degree of exposure to environmental factors.