P53, a 53 kDa protein, binds to a DNA consensus sequence, the p53 response element, and regulates normal cell cycle events by activating transcription of genes involved either in progression through the cell cycle, or causing arrest in G1 when the genome is damaged. In most transformed and tumor cells the concentration of p53 is increased 5-1000 fold over the concentration in normal cells, principally due to the increased half-life (4 hrs) compared to that of wild-type (20 min). p53 localizes in the nucleus, but is detectable at the plasma membrane during mitosis and certain mutations also modulate cytoplasmic/nuclear distribution. p53 downregulates Bcl-2 expression and upregulates Bax expression, but may not always be necessary for apoptosis.Synonyms: Cellular tumor antigen p53, NY-CO-13, Phosphoprotein p53, Tumor suppressor p53