Dicordant retinoblatoma in monozygotic twin

PURPOE: We report cae of dicordant retinoblatoma in twin confirmed to be monozygotic by DNA analyi. METHOD: Twin A demontrated evere, bilateral, multifocal retinoblatoma, which wa recalcitrant to external beam irradiation and chemoreduction. Twin B ha not demontrated retinoblatoma. DNA analyi wa performed with polymorphic microatellite marker to confirm monozygoity. ingle-tranded conformation polymorphim and outhern blot analyi of the retinoblatoma gene were performed. REULT: Molecular genetic analye confirmed monozygoity but failed to dicloe a retinoblatoma gene mutation in either twin. CONCLUION: The extreme phenotypic dicordance may bet be explained by an unidentified, potzygotic retinoblatoma gene mutation in early embryonic development of the affected twin.