Figure 1. Regulation of the G1 to S phase transition by the retinoblastoma suppressor protein (pRB). G0, G1, S, G2, and M refer to the quiescence, first gap, DNA synthesis, second gap, and mitosis phases of the cell cycle. CDK refers to cyclin-dependent kinases. CDC-2 refers to cell cycle control-2. Phosphorylated pRB is represented as pRB-P. Briefly, pRB-E2F transcriptional repression of genes regulating DNA synthesis is released by phosphorylation of pRB, allowing for cell cycle progression from G1 to the S phase.