Studies of naturally occurring mutations of the RB1 tumor suppressor gene have indicated that the E1A/T antigen-binding domain is important for pRb function. Mutations engineered within the C-terminal 135 amino acids of pRb also abrogate its growth-suppressive function during the G1 interval of the cell cycle. Both the pRb E1A/T antigen-binding domain and the C-terminal domain are required for interaction with the E2F transcription factor. A series of mutated pRb proteins has been used to define the C-terminal sequences which determine E2F binding, adenovirus E2 promoter inhibition, and negative growth control. Deletion of the C terminus to residue 870 allowed full pRb function, while further deletion to residue 841 inactivated pRb in each assay. Amino acid sequences immediately C-terminal to the E1A/T antigen-binding domain were absolutely required for pRb activity. Mutations which prevented pRb from interacting with E2F also eliminated pRb-mediated E2 promoter repression and inactivated the ability of pRb to suppress cell growth.