Fluorescent in situ hybridization (FISH) studies of human preimplantation embryos have demonstrated a high proportion of chromosomal mosaicism. To investigate the different timings and nature of chromosomal mosaicism, we developed single cell multiplex¯uorescent (FL)-PCR to distinguish meiotic and mitotic cell division errors. Chromosome 21 was investigated as the model chromosome as trisomy 21 (Down's syndrome) represents the most common chromosomal aneuploidy that reaches live birth. Sister

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... astomeres from a total of 25 chromosome 21 aneuploid embryos were analysed. Of these, 13 (52%) comprised cells with concordant DNA ®ngerprints indicative of meiotic non-disjunction errors. The remaining 12 (48%) aneuploid embryos comprised discordant sister blastomere allelic pro®les and thus were mosaic. Errors at all stages including metaphase (MI) (12%) and ®rst (38%), second (31%) and third (19%) mitotic cleavage divisions were identi®ed from the types and proportion of different allelic pro®les. In addition, three embryos showed combined meiotic and mitotic cell division errors including non-disjunction and anaphase lag, suggesting that diploid cells had resulted from an aneuploid zygote. However, the majority of the mosaic aneuploid embryos showed mitotic gains and losses from a diploid zygote occurring prior to the activation of the embryonic genome. Allelic pro®ling of amniocytes from 15 prenatal diagnosis samples displayed only meiotic errors. There appears to be a large difference between the proportion of mosaic mitotic-derived trisomy 21 embryos and fetuses. These ®ndings indicate that mosaic mitotic error of chromosome 21 is associated with non-viability.