Chimerism Involving Arb1pathogenic Variant In Monochorionic Dizygotic Twins With Twin-Twin Transfusion Syndrome

We report the first case of blood chimerism involving a pathogenicRB1variant in naturally conceived monochorionic-dizygotic twins (MC/DZ) with the twin-twin-transfusion syndrome (TTTS), presumably caused by the exchange of stem-cells. Twin A developed bilateral retinoblastoma at 7 months of age. Initial genetic testing identified a de novoRB1pathogenic variant, with a 20% allelic ratio in both twins' blood. Subsequent genotyping of blood and skin confirmed dizygosity, with the affected twin harboring theRB1pathogenic variant in skin and blood, and the unaffected twin carrying the variant only in blood.