45,X/46,XX mosaicism in discordant monozygotic twins

Monozygotic twins discordant for Turner's syndrome were both mosaic for 45,X/46,XX in the blood with the same low frequency of 2% to 3% 45,X cells. However, the fibroblasts of the abnormal girl were all uniformly 45,X whereas her normal twin had only 46,XX cells. Monozygosity was confirmed by genetic markers, chromosome variants, and a single monochorionic placenta with a shared vascular circulation. The mechanism by which this disparate pair developed from a single zygote is suggested.