Discordant retinoblastoma in monozygotic twins

PURPOSE: We report cases of discordant retinoblastoma in twins confirmed to be monozygotic by DNA analysis. METHODS: Twin A demonstrated severe, bilateral, multifocal retinoblastoma, which was recalcitrant to external beam irradiation and chemoreduction. Twin B has not demonstrated retinoblastoma. DNA analysis was performed with polymorphic microsatellite markers to confirm monozygosity. Single-stranded conformation polymorphism and Southern blot analysis of the retinoblastoma gene were performed. RESULTS: Molecular genetic analyses confirmed monozygosity but failed to disclose a retinoblastoma gene mutation in either twin. CONCLUSIONS: The extreme phenotypic discordance may best be explained by an unidentified, postzygotic retinoblastoma gene mutation in early embryonic development of the affected twin.