Genetics and cytogenetics of retinoblastoma.

Retinoblastoma tumor formation is initiated by loss of function of both alleles at the RB1 locus on chromosome 13. In nonhereditary retinoblastoma (60% of patients), both mutations occur during retinal development. In hereditary retinoblastoma (40% of patients), tumor formation is caused by one germline and one somatic mutation. The RB1 gene encodes a nuclear protein that arrests progression through the G1 phase of the cell cycle. In the absence of intact RB1 protein, unscheduled cell proliferation occurs. Genes on chromosomes 1 and 6, which have not yet been identified, appear to be involved in later stages of tumorigenesis.