Master or slave: the complex relationship of RBP2 and pRb.

The retinoblastoma protein or its regulators are altered in most human cancers. Although commonly thought of as solely a repressor of E2F-dependent transcription and cell cycle progression, pRb has gained notoriety in recent years as a key actor in cellular differentiation programs. In the June issue of Molecular Cell, Benevolenskaya et al. report that a long-known but poorly understood pRb interactor, RBP2, acts as an inhibitor of differentiation contributing to pRb's role as a coordinator of differentiation and cell cycle exit. Loss of pRb may unleash RBP2, maintaining cells in a poorly differentiated progenitor state that is prerequisite to tumor formation.