The p53 protein plays a central role in modulating cellular responses to cytotoxic stresses by contributing to both cell-cycle arrest and programmed cell death. Loss of p53 function during tumorigenesis can lead to inappropriate cell growth, increased cell survival, and genetic instability, p53 gene mutations occur in approximately half of all malignancies from a wide range of human rumors. In some tumor types, these p53 mutations are associated with poor prognosis and treatment failure, Based on these insights, new approaches are being developed to prevent diagnose, and treat cancer. J Clin Oncol 16:3158-3168. (C) 1998 by American Society of Clinical Oncology.