P21 protein, also known as p21^Cip1/Waf1, is a crucial regulator of the cell cycle. It is encoded by the CDKN1A gene and functions primarily as a cyclin-dependent kinase inhibitor. By inhibiting these kinases, P21 helps control cell division, ensuring that cells do not proliferate uncontrollably, which is important for maintaining normal cellular function and preventing cancer. P21 is also involved in cellular responses to stress and DNA damage. When cells experience damage, P21 can be activated to halt the cell cycle, allowing time for repair mechanisms to fix the issues. This protective role makes P21 an important factor in tumor suppression and overall cellular health.