Nonhereditary retinoblastoma (55%) also develops because of biallelic inactivation of the RB1 gene in approximately 97% of patients; however, a small percentage of patients with sporadic disease (approximately 1.5%) show MYCN amplification as the initiating event,6 whereas the disease of the remaining percentage (approximately 1.5%) remains unexplained. This evidence concerns the gene MYCN and retinoblastoma.