p21 (also known as CDKN1A) is a member of the cell cycle inhibitor family, primarily regulating the cell cycle by inhibiting cyclin-dependent kinases (CDKs). It features a CDK binding domain. p21 is mainly located in the nucleus, responding to DNA damage and other cellular stress signals, activated through the p53 signaling pathway. The expression of p21 is tissue-specific, particularly upregulated in response to cellular stress or damage. Mutations or dysregulation of its expression are associated with the development of various cancers.