<P> p53 expression can be stimulated by UV light, which also causes DNA damage . In this case, p53 can initiate events leading to tanning . </P> <P> The p21 protein binds directly to cyclin - CDK complexes that drive forward the cell cycle and inhibits their kinase activity thereby causing cell cycle arrest to allow repair to take place . p21 can also mediate growth arrest associated with differentiation and a more permanent growth arrest associated with cellular senescence . The p21 gene contains several p53 response elements that mediate direct binding of the p53 protein, resulting in transcriptional activation of the gene encoding the p21 protein . </P> <P> Levels of p53 play an important role in the maintenance of stem cells throughout development and the rest of human life . </P> <P> p53 is maintained at low inactive levels in human embryonic stem cells (hESCs). This is because activation of p53 leads to rapid differentiation of hESCs . Studies have shown that knocking out p53 delays differentiation and that adding p53 causes spontaneous differentiation, showing how p53 promotes differentiation of hESCs and plays a key role in cell cycle as a differentiation regulator . When p53 becomes stabilized and activated in hESCs, it increases p21 to establish a longer G1 . This typically leads to abolition of S - phase entry, which stops the cell cycle in G1, leading to differentiation . p53 also activates miR - 34a and miR - 145, which then repress the hESCs pluripotency factors, further instigating differentiation . </P>

Why are p53 and rb proteins called checkpoint proteins