RB1 and cancer: Although ID-1 has been shown to interact with various cell cycle regulators (Alani et al, 2001; Ohtani et al, 2001), ID-1 seems to be an essential factor in the promotion of G1/S cell cycle transition in certain cancers by inactivating of p16 and increasing of CDK4 activity/RB (Polsky et al, 2001; Ouyang et al, 2002b; Singh et al, 2002).