The occurrence of monochorionic diamniotic twins with sex discordance is a very rare phenomenon. We present a case of spontaneously conceived gender-discordant monochorionic diamniotic twins born to a 23-year-old female, both twins demonstrating similar blood karyotype 45,X/46,X, idic(Y) and a novel 99 kb mutation at 3p24.3 involving exons 15–16 of transcript NM_001134381.1 of the Tre-2/Bub2/Cdc16 Domain Family Member 5 (TBC1D5) gene. The male twin showed no anatomic abnormalities and pelvic ultrasound revealed descended gonads. The female twin had a horseshoe-shaped kidney, normal uterus, and intra-abdominal gonads. The blood karyotype and microarray studies revealed similar distribution of X and isodicentric Y chromosome along with a novel genetic mutation which has not been previously reported. Our case findings not only report Turner syndrome mosaicism with a novel genetic mutation but also stress the importance of clinical follow-up of twins in order to evaluate the functional abnormalities associated with isodicentric Y chromosomes including germ cell tumors.
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