Cavenee et al. (1983) further delineated the recessive nature of the gene predisposing to retinoblastoma by using chromosomal molecular markers to show that tumorigenesis coincides with the loss of heterozygosity around the RB1 locus resulting from mitotic nondisjunction or recombination [11,12] (Figure 1D). This evidence concerns the gene RB1 and retinoblastoma.