TP53 and cancer: Inactivation of p53 is the key event in most of malignancies, and mutation of p53 has been thought to be responsible for tumorigenesis in most cancers [34]; however, recent studies showed that functional inactivation of p53 by alteration of proteasomal p53 regulators such as MDM2 also contributes to escape from p53-regulated growth arrest and cell death.