p107 is a cellular protein that forms specific complexes with adenovirus E1A and SV40 large T antigen (T). The genetics of the p107-T/E1A interaction as well as other features of this protein suggests that p107 shares functional properties with the tumor suppressor product, RB. A partial cDNA for human p107 has been cloned. Its sequences map to 20q11.2 and encode a 936 residue protein. Comparison analysis of the p107 protein sequence reveals a major region of RB homology extending over 564 residues. This region in RB is essential to its growth-controlling function. Sequences outside of these two regions are largely unique to each protein. The p107 and RB homology regions can independently bind to T and E1A. Thus, these two proteins display similarities of structure that may, at least in part, explain their known functional similarities and suggest a generic function for p107 in cell cycle regulation.