Determination of the sexual phenotype in a child with 45,X/46,X,Idic(Yp) mosaicism: Importance of the relative proportion of the 45,X line in gonadal tissue

We report on a girl who, despite her 45,X/46,X,der(Y) karyotype, showed no signs of virilization or physical signs of the Ullrich–Turner syndrome (UTS), except for a reduced growth rate. After prophylactic gonadectomy due to the risk of developing gonadoblastoma, the gonads and peripheral blood samples were analyzed by fluorescence in situ hybridization (FISH) and polymerase chain reaction (PCR) to detect Y‐specific sequences. These analyses allowed us to characterize the Y‐derived chromosome as being an isodicentric Yp chromosome (idic(Yp)) and showed a pronounced difference in the distribution of the 45,X/46,X,idic(Yp) mosaicism between the two analyzed tissues. It was shown that, although in peripheral blood almost all cells (97.5%) belonged to the idic(Yp) line with a duplicated SRY gene, this did not determine any degree of male sexual differentiation in the patient, as in the gonads the predominant cell line was 45,X (60%). © 2006 Wiley‐Liss, Inc.