The retinoblastoma tumor suppressor gene (RB1) and its relatives, p107 and p130, encode a family of proteins that share several properties, including the capacity to regulate E2F-dependent transcription and inhibit cell-cycle progression. Although RB1 inactivation is widely implicated in human cancer, the growth regulatory functions of p107 and p130, and the functional relationships within the gene family, are emerging only recently. Here we review studies of RB1 gene family function, with emphasis on in vivo experiments that explore shared and distinct functions within this family.