We report an example of four generation familial retinoblastoma in which there are three distinct categories of RB gene expression: frank retinoblastoma, unilateral or bilateral; retinoma; and no visible evidence of retinal pathology other than normal degeneration with age. Two large sibships derived from matings informative for RB and EsD provide strong confirmatory evidence for tight linkage between these loci (P = 0.0002), and thus assignment of RB to chromosome 13q14. There is a striking difference (P less than 2%) in RB penetrance between the two principal generations, which suggests that an additional epistatic, host-resistance gene may also be segregating within the family.