P21

P21 is an example of a cyclin-dependent kinase inhibitor (CKI). P21 protein inhibits the activity of cyclin-CDK2, -CDK1, and -CDK4/6 complexes, and therefore functions acts as a modulator of cell cycle in turn determining wheteher progression at G1 and S phase will occur.

Reference:

Harper JW, Adami GR, Wei N, Keyomarsi K, Elledge SJ (November 1993). "The p21 Cdk-interacting protein Cip1 is a potent inhibitor of G1 cyclin-dependent kinases". Cell. 75 (4): 805–16. doi:10.1016/0092-8674(93)90499-G. PMID 8242751