p53 protein plays a central role in cellular responses, including cell-cycle arrest and cell death in response to DNA damage such as that caused by many chemotherapeutic agents and radiation used in cancer therapy. p53 dysfunction can induce abnormal cell growth, increased cell survival, genetic instability, and drug resistance. p53 mutations occur in approximately half of human cancers. They have been found in about 20% of patients with aggressive B-cell lymphoma and are associated with poor treatment outcome and poor prognosis. The purpose of this review is to provide an outline of the biological functions of p53 and assess the role of p53 in clinical settings.