p21WAF1/CIP1 gene is known for a most important cell cycle regulator as well as its roles in appoptosis and differentiation. This review focuses on p21WAF1/CIP1 gene functions and its association with carcinogenesis. Better understanding of the structure and function of p21WAF1/CIP1 gene may help to comprehend molecular mechanisms of cancers and to facilitate diagnosis and treatment of malignancy.