RB1 and neoplasm: The versions of Rb examined included the wild type (RbWT), the tumor-derived mutants RbΔ21 and RbΔ22 [39], a low penetrance mutant (Rb661W) that retains partial tumor suppressor function but lacks the ability to regulate E2F1 [40], and a mutant (Rb76t) with a premature termination codon at amino acid 76 [41].