FIG. 10.
Summary model of HBP1 and RB functions in differentiation. We postulate that muscle differentiation can be divided into general cell cycle exit (G0) and tissue-specific gene expression (terminal differentiation) coordinated by the RB family (RB and p130) and their targets, such as HBP1. A high p130/HBP1 or low RB/HBP1 ratio may favor the cell cycle exit but act as a negative signal for terminal differentiation. This transient suspension of differentiation may eventually be relieved by activation of RB, resulting in a high RB/HBP1 ratio. This complex regulatory mechanism may be an effective means for ensuring fidelity in differentiation by ensuring that only viable and arrested cells proceed to the irreversible expression of genes that specify individual tissue phenotypes.