RB1 and retinoblastoma: Hereditary patients carry a deleterious germ line mutation in one RB1 allele and therefore only require one somatic mutation in the wild type RB1 allele for retinoblastoma to develop while non-hereditary patients require two somatic mutations in RB1. However, while bi-allelic inactivation of RB1 can cause benign retinoma lesions, additional genetic alterations can be required for progression to retinoblastoma [5].