The distinct mutations within the RB1 gene associated with low penetrance retinoblastoma include a point mutation at codon 661 of exon 20 [9,10], a 3-bp deletion in exon 16 that results in the deletion of Asn480 [9], a 4-kb deletion involving exons 24 and 25 [11], and a splicing mutation at the last base of exon 21 [12]. Here, RB1 is linked to retinoblastoma.