pRb, p107 and p130 are important regulators of cell cycle and have extensive overlapping functions; however, only Rb has been shown to be a bone fide tumor suppressor. Defining the overlapping versus distinct pocket protein functions is therefore an important step to understanding the unique role of Rb. Using lung as a model, the present studies demonstrate that pocket proteins are important not only in regulating cell cycle and survival but also in cell lineage specification. An inducible lung-specific Rb knockout strategy was used to demonstrate that Rb is specifically required for restricting neuroendocrine cell fate despite functional compensation for Rb deficiency in other cell types. Ablation of total Rb family function resulted in opposing effects in specification along distinct cell lineages, providing evidence that pocket proteins inhibit neuroendocrine cell fate while being required for differentiation in other cell types. These findings identify a novel role for pocket proteins in cell fate determination, and establish a unique cell lineage-specific function for Rb that explains, at least in part, why Rb and p16 are inactivated in phenotypically distinct carcinomas.