During early M phase, when mitosis begins, the cell is awash with active cyclin–Cdk complexes. Those S-Cdks and M-Cdks must be disabled by the end of M phase to allow the cell to complete division and to prevent it from initiating another round of division without spending any time in G 1 .

To usher a cell from the upheaval of M phase to the relative tranquility of G 1 , the cell-cycle control machinery must inactivate its inventory of S-Cdk and M-Cdk.

It does so in several ways: by eliminating all of the existing cyclins, by blocking the synthesis of new ones, and by deploying Cdk inhibitor proteins to muffle the activity of any remaining cyclin–Cdk complexes. The use of multiple mechanisms makes this system of suppression robust, ensuring that essentially all Cdk activity is shut down.

This wholesale inactivation resets the cell-cycle control system and generates a stable G 1 phase, during which the cell can grow and monitor its environment before committing to a new round of division.