Nucleic acids that encode novel polypeptides, designated in the present application as "BOG" (B5T Over-expressed Gene) are provided. BOG binds to pRb and is over-expressed in a number transformed rat liver epithelial (RLE) cell lines resistant to the growth inhibitory effect of TGF-.beta.1 as well as in primary liver tumors. Compositions including BOG chimeras, nucleic acids encoding BOG and antibodies to BOG are also provided. Methods of using BOG to modulate pRb-protein interactions and to alter cellular phenotype are further provided.