This entry represents the cyclin-dependent kinase inhibitor 1 (CDKN1A, also known as p21) from animals. It is a direct transcriptional target of p53 and may repress apoptosis []. It mediates several important physiological effects of p53, including DNA-damage-induced cell cycle checkpoints [, ]. p21 plays a critical role in regulating cell growth and the cell response to DNA damage[].