The pocket protein family consists of three structurally and functionally related proteins, Rb (retinoblastoma), p107, and p130. This family of tumor suppressors function to regulate important cellular transcription factors, such as the E2F family. The E2F proteins regulate the expression of genes whose products are important for cell cycle progression. The inactivation Rb is catalyzed by CDK phosphorylation thereby releasing E2F during the G1-S phase cellular progression. Unchecked inactivation of Rb in G1 phase has been indicated as a universal mechanism underlying cellular transformation . While Rb has been the most studied among the pocket proteins, p107 and p130 have also been shown to be key regulators of E2F. Several studies have also provided evidence that p107/p130 provide different functions in E2F regulation than does Rb. Rb, p107, and p130 each contain a conserved 'A/B pocket', which is the target of several viral oncoproteins, namely SV40 large T-antigen and adenovirus E1A. There are two isoforms.