The retinoblastoma gene product, pRB, regulates cell proliferation by binding to and inhibiting the activity of key growth promoting proteins. Several cellular proteins have been shown to bind directly to pRB and the genes encoding a number of them have been isolated. The protein product of one of these genes is the transcription factor E2F. We have now isolated cDNA clones that contain the full-length coding sequence of two other proteins, RBP1 and RBP2, cloned originally by their interaction with pRB. The products of the RBP1 and RBP2 genes are ubiquitously expressed, large (200 kDa for RBP1 and 195 kDa for RBP2) nuclear phosphoproteins with structural motifs that suggest a role in transcriptional regulation. In addition we have been able to identify complexes of pRB and RBP1 in vivo that are dissociated in the presence of purified human papillomavirus E7 protein.