FIGURE 1.

Features of the mammalian cell cycle. Mammalian cell cycle is tightly regulated and can be artificially divided into four distinct phases (G1, S, G2, and M) according to their specific characteristics. G0 phase is usually used to describe cells that have exited the cell cycle and become quiescent. The progression of the mammalian cell cycle is precisely controlled by CDKs, cyclins, and CKIs. Checkpoints could ensure the processes at each phase of the cell cycle have been accurately completed before entering into the next phase.