Schematic representation of pRB, p107 and p130 open reading frames. (A) The central feature of RB-family proteins is the pocket domain. It was originally defined as the minimal domain necessary to bind to viral oncoproteins such as simian virus TAg through their LXCXE motif, and is denoted as the 'small pocket' in this diagram. The 'large pocket' is the minimal growth suppressing domain of RB-family proteins and it is capable of binding E2F transcription factors as well as viral proteins. (B) Comparison of open reading frame structures of each of the pocket proteins. Note the additional features found in the p107 and p130 proteins, the kinase inhibitory site, the cyclin binding site, and the insertion in the B-domain of the pocket. These provide the most obvious differences between pRB and its relatives p107 and p130.