TP53 and neoplasm: Despite this, loss of function mutations in the CDKN1A gene, which encodes p21, are extremely rare in tumors (40), suggesting that p21 may not be a bona fide tumor suppressor, but rather exerts anti-tumor effects by synergizing with other tumor suppressor pathways (e.g., APC, ATM, and p53) or counteracting the activity of oncogenes, such as HRAS and Myc (26).