CDKN1A and neoplasm: Protein p21 (CIP1) or cyclin-dependent kinase inhibitor 1A (CDKN1A) is another important tumor suppressor controlling cell cycle checkpoint mechanisms that inhibits cyclins, CDKs, or their complexes (cyclin-CDK2, cyclin-CDK1, and cyclin-CDK4,6) (Al Bitar and Gali-Muhtasib, 2019).