RB1 and retinoblastoma: RB occurs in a non-hereditary form with monocular involvement due to a somatic mutation of both RB1 alleles and, in a hereditary form in which the ocular involvement might be single or bilateral with a first mutation of the RB1 allele occurring in the germline and subsequently taking place the other somatic mutation (Knudson, 1971; Comings, 1973).