P53, or tumor protein 53, is a well known tumor suppressor protein that is encoded by the gene TP53 in humans. Other aliases include tumor antigen p53, antigen NY-CO-13, LFS1, and transformation related protein 53. The full length protein is 393 amino acids in length that corresponds to a mass of 43.7 kDa. Due to a high number of prolines, the protein appears to be 53-kDa when run on SDS-PAGE. There are as much as 15 reported p53 isoforms. Antibodies against p53 may be useful in studying its many roles in apoptosis, tumorigenesis, cell cycle, and longevity.