This book is about the p53 gene, one of the most frequently mutated or deleted genes in human cancers. The frequent occurrence of inactivated p53 implicates this gene product in the genesis of many human cancers. The p53 gene can suppress the growth of cancer cells and the transformation process by oncogenes. The p53 protein is a transcription factor that can repress or activate promoters containing one of three p53 DNA-binding motifs. The activity of p53 is regulated by phosphorylation and other transcription factors. Replacement of the p53 function or restoration of the p53 biochemical pathway is a focus of gene therapy.