p21, also known as p21Cip1 , p21Waf1, cyclin-dependent kinase inhibitor 1, or CDK-interacting protein 1, functions to regulate cell cycle progression at G1 by inhibiting the activity of Cyclin-CDK2 or -CDK4 complexes. This cyclin-dependent kinase inhibitor is expressed in all adult human tissues, and decreased expression of p21 is linked to poor prognosis in a number of carcinomas including gastric carcinoma, non-small cell lung carcinoma and thyroid carcinoma. p21 is also associated with favourable prognosis in several tumors.