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Purpose: To compare the frequency and the degree of mosaicism in human embryos from Robertsonian translocation (RT) t(13;14) carriers, with embryos from karyotypically normal IVF patients. Methods: FISH analysis of embryos from PGD cycles for RT t(13;14), with probes for chromosomes 13, 14, and 18 (Group I) and of embryos from karyotypically normal IVF patients with probes for chromosomes 13, 18, 21, X, and Y (Group II). Results: The incidence of abnormal mosaic embryos was significantly higher in group I (38/51) as compared with group II (6/45) (χ2: P < 0.01). Furthermore, in group I the percentage of diploid cells per embryo was lower for chromosome 13 and 14 in comparison with 18, while in group II no differences were observed between the five chromosomes analyzed. Conclusions: RT induces a high frequency of mosaicism specifically for the chromosomes implicated in the translocation; the analysis by FISH of two blastomeres is strongly recommended for these patients.