Figure 1. The S phase (DNA synthesis) of the cell cycle is triggered by the action of cyclin/cdk pairs, which phosphorylate pocket proteins such as Rb (the retinoblastoma gene product). This phosphorylation releases Rb from a complex of transcription factors (E2F/DP), which then induce the synthesis of genes required for S phase. C/EBP alpha inhibits the action of the E2F/DP heterodimer, thus preventing cell division.