p53 is a 53 kD protein that forms tetramers and functions as a tumor suppressor and transcriptional activator of genes that inhibit growth and/or invasion, cell cycle checkpoint after irradiation, DNA repair, apoptotic induction, signal transduction, and cell adhesion. This protein is localized to the nucleus when activated and can be upregulated by genotoxic or other cellular stresses. p53 is modified by phosphorylation, acetylation, ribosylation, ubiquitination, and sumoylation; ubiquination targets p53 for degradation via mdm2. This protein interacts with a variety of proteins including mdm2, mdmx, topoisomerase I, PML3, Bcl-XL, Bcl-2, Chk1, JNK, p38, HIPK2, CK2, DNA-PK, p300/CBP, PCAF, PARP1, and HDAC1-3. Mutant p53 associates with p63 and p73.