FIGURE 19-10 An overview of how CDKs regulate cell cycle progression. Cells harbor different types of CDKs that initiate different events of the cell cycle. Importantly, the CDKs are active only in the stages of the cell cycle they trigger. G₁/S phase CDKs are active at the G₁–S phase transition to trigger entry into the cell cycle. S phase CDKs are active during S phase and trigger S phase. Mitotic CDKs are active during mitosis and trigger mitosis. The anaphase-promoting complex or cyclosome (APC/C) ubiquitin-protein ligase catalyzes two key cell cycle transitions by ubiquitinylating proteins, hence targeting them for degradation. APC/C initiates anaphase and exit from mitosis.