P53 (also known as protein 53 or tumor protein 53), is a tumor suppressor protein that in humans is encoded by the TP53 gene.

P53 is often described metaphorically, either as “the guardian of the genome”, or “the guardian angel gene”, or the “gene of cell’s conscience”,the “master watchman” referring to its role in conserving stability by preventing genome mutation. As it binds to damaged DNA and repair it before its replication.

P53-DNA  complex

Mutant p53 was compared to a “fallen angel “,referring to its role in conserving stability by preventing genome mutation. 

The name p53 is in reference to its apparent  molecular mass.It runs as a 53- Kilodalton (kDa) protein on SDS-PAGE. But, p53's actual mass is only 43.7 kDa. due to the high number of proline residues in the protein, which slow its migration on SDS-PAGE. p53 is a nuclear phosphoprotein which acts as a tumor suppressor.

The p53 gene like the Rb gene, is a tumor suppressor gene, i.e., its activity stops the formation of tumors. If a person inherits only one functional copy of the p53 gene from their parents, they are predisposed to cancer and usually develop several independent tumors in a variety of tissues in early adulthood.

The tumour suppressor protein p53 is localized in the cell nucleus where it serves to initiate cellular responses to a variety of stresses. This gene responds to diverse cellular stresses to regulate target genes that induce cell cycle arrest, apoptosis, senescence, DNA repair, or changes in metabolism.

p53 protein is expressed at low level in normal cells and at a high level in a variety of transformed cell lines, where it's believed to contribute to transformation and malignancy.

Mutants of p53 that frequently occur in a number of different human cancers fail to bind the consensus DNA binding site, and hence cause the loss of tumor suppressor activity.