The retinoblastoma susceptibility gene (RB), the first identified human tumor suppressor gene, has been shown to be directly involved in the genesis of a variety of human cancers. RB is actually one of a family of three closely related genes including p107 and p130. Many elegant biochemical studies have demonstrated that RB is a critical component of the cell cycle regulatory machinery and have characterized the downstream effectors which the RB gene product regulates. More recent advances have demonstrated that the function of RB and RB-related genes is positively and negatively regulated by an intricate network of cell cycle regulatory proteins, some of which have also been implicated as tumor suppressor genes. Despite the detailed understanding of these biochemical and genetic pathways, the full function of genes in the RB pathway in the context of a whole organism is only now being addressed. Using gene knockout technology, it is now known that RB, and RB-related proteins p107 and p130, have important functions during early mouse development. Furthermore, despite its ubiquitous expression, RB has tissue- and cell-type specific effects which account for its function as a tumor suppressor but may also be independent of its role as a cell cycle regulator. Analysis of mice lacking regulatory genes upstream of RB and effector genes downstream of RB have confirmed that other genes in this pathway have tissue-specific effects on development and tumor susceptibility in mice.