Alternative definition : KEGG: Mitotic cell cycle progression is accomplished through a reproducible sequence
of events, DNA replication (S phase) and mitosis (M phase) separated temporally by
gaps known as G1 and G2 phases. Cyclin-dependent kinases (CDKs) are key regulatory
enzymes, each consisting of a catalytic CDK subunit and an activating cyclin subunit.
CDKs regulate the cell's progression through the phases of the cell cycle by modulating
the activity of key substrates. Downstream targets of CDKs include transcription factor
E2F and its regulator Rb. Precise activation and inactivation of CDKs at specific
points in the cell cycle are required for orderly cell division. Cyclin-CDK inhibitors
(CKIs), such as p16Ink4a, p15Ink4b, p27Kip1, and p21Cip1, are involved in the negative
regulation of CDK activities, thus providing a pathway through which the cell cycle
is negatively regulated.;