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. Copyright (C) 1999 Elsevier Science Inc.
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