p21WAF1, also known as cyclin-dependent kinase inhibitor 1 or CDK-interacting protein 1, is a protein that in humans is encoded by the CDKN1A gene located on chromosome 6 (6p21.2). May be the important intermediate by which p53/TP53 mediates its role as an inhibitor of cellular proliferation in response to DNA damage. Binds to and inhibits cyclin-dependent kinase activity, preventing phosphorylation of critical cyclin-dependent kinase substrates and blocking cell cycle progression. Functions in the nuclear localization and assembly of cyclin D-CDK4 complex and promotes its kinase activity towards RB1. At higher stoichiometric ratios, inhibits the kinase activity of the cyclin D-CDK4 complex. p21 is expressed in all adult human tissues. In tumors, the expression of p21 has been studied by immunohistochemical methods in a wide range of human tumors, such as gastric carcinoma, non-small cell lung carcinoma, and thyroid carcinoma. The expression of p21 is associated with favorable prognosis in various tumors.