The retinoblastoma protein as a fundamental mediator of growth and differentiation signals.

The retinoblastoma gene (RB) is the prototype of the tumor suppressor genes, which play critical roles in the genesis of cancer in humans. Mouse models created through gene knock-out and transgenic methods were established for exploring and manipulating RB in vivo. These models and several other pieces of evidence have shown that the retinoblastoma protein (Rb) plays dual roles in gating cell cycle progression and promoting cellular differentiation. The molecular mechanisms involved in these roles are becoming more obvious in some biological systems: Rb sequesters the transcription factor of E2F to regulate entry of cell cycle but enhances the activities of another class of the transcription factors, exemplified by NF-IL6, to initiate terminal cellular differentiation. Thus, the Rb protein can serve as a mediator for extracellular signals of growth or differentiation. The fundamental question of why only limited cell types are susceptible to tumor formation when Rb expression is lost, however, remains unanswered at present.