This year the p53 protein, also known as "guardian of the genome", turns twenty five years old. During this period the p53 knowledge have changed from an initial pro-oncogene activity to the tumorsupressor p53 function. p53 is activated upon stress signals, such as gamma irradiation, UV, hypoxia, virus infection, and DNA damage, leading to protection of cells by inducing target genes. The molecules activated by p53 induce cell cycle arrest, DNA repair to conserve the genome and apoptosis. The regulation of p53 functions is tightly controlled through several mechanisms including p53 transcription and translation, protein stability, post-translational modifications, and subcellular localization. In fact, mutations in p53 are the most frequent molecular alterations detected in human tumours. Furthermore, in some degenerative processes, fragmentation and oxidative damage in DNA take place, and in these situations p53 is involved. So, p53 is considered a pharmacological target, p53 overexpression induces apoptosis in cancer and its expression blockage protects cells against lethal insults.