The methylation status of the promoter region of the cell cycle gene P14ARF was studied in the extraembryonic mesoderm and in the chorion cytotrophoblast of 46 human spontaneous abortuses with chromosomal mosaicism. Aberrant methylation of alleles of this gene was revealed for the first time in placental tissues of 9% of embryos. The identified epimutations were found to be characteristic of embryos with aneuploid cell clones of postzygotic origin. It is suggested that epigenetic inactivation of loci responsible for the regulation of cell division and for segregation of chromosomes is associated with the occurrence of mosaic forms of the karyotype at early stages of human embryonic development.