CDKs are inactive in their monomeric form. They require association with a specific cyclin to become active. Once activated, CDKs phosphorylate target proteins to drive the cell through different phases of the cell cycle, such as the G1, S, G2, and M phases. For example, CDK1 pairs with cyclin B to initiate mitosis, while CDK2 pairs with cyclin E to promote the G1/S transition.
