CDKN1A and neoplasm: Given that senescent tumor cells arrest in the G1 or G2/M phase of the cell cycle, it stands to reason that there may be variable expression of some of the cell cycle protein regulators (in G1 phase, G1/S checkpoint, G2 phase, and GS/M checkpoint) such as p16INK4a, p21Cip1/Waf1, and p27 to achieve and maintain growth arrest.