MONOZYGOTIC TWINS WITH 45,X 46,XY MOSAICISM DISCORDANT FOR PHENOTYPICSEX/

We report on two sets of monozygotic twins (MZTs) discordant for pheno typic sex ascertained at birth when the female twin was noted to have signs of the Ullrich-Turner syndrome, Cytogenetic investigations on th e female of the first pair showed 45,X/46,XY mosaicism in lymphocytes but fibroblasts grown from two skin biopsies at separate sites and fro m gonadal tissue showed only 45,X cells. The male showed mosaicism in both blood lymphocytes and skin fibroblasts. In the second family, bot h twins also showed mosaicism in lymphocytes, The female had a 45,X ka ryotype in fibroblasts from skin and gonadal tissue, but in contrast t o the first family, the male twin had a normal male karyotype in fibro blasts from skin biopsy and from connective tissue adjacent to the vas deferens. Discordant phenotypic sex in monozygotic twins is rare. As in our cases, the nine previously reported sets of MZTs all had mosaic ism for sex chromosome abnormalities. A mitotic error leading to the l oss of a Y chromosome prior to, accompanying, or following the twinnin g process would account for the reported combinations of karyotypes. ( C) 1998 Wiley-Liss, Inc.