Cyclin and the cyclin dependent kinases (CDKs) are the principle regulators of cell cycle progression through activation of cell cycle checkpoints.
These are the core mechanism which drives the irreversible transitions that must be successfully negotiated for cells to successfully navigate the cell cycle and proliferate.
During G1 phase there are a series of critical criteria that must be met for cells to proceed to S phase.
These criteria and those necessary for cells to reenter the cell cycle through G1 phase, are governed by a subset of the cyclin/CDKs.
They are specifically activated during G1 phase, and are responsible for rendering cells permissive for irreversible entry into S phase and continued proliferation or, alternatively, retirement from the cell cycle to become senescent, quiescent or to terminally differentiate.