Retinoblastoma

Retinoblastoma (McKusick 180200) is the most
frequent tumor of the eye in infancy and early
childhood. It occurs in 1 of 15000–18000 live
births. This tumor results from loss of function
of both alleles of the retinoblastoma gene RB1.
Tumor initiation is preceded by two steps as A.
Knudson predicted in 1971 in his “two-hit” hypothesis
(tumor suppressor gene, p. 318). The
first predisposing mutation in one allele may
occur either in a retinoblast, an undifferentiated
retinal cell in the developing embryo, or
in the germline. The other allele is inactivated
by a second mutation.