PathwayStep4457 Transcriptional activation of cell cycle inhibitor p21 Transcriptional activation of p53 responsive genes Pubmed7664346 Pubmed8242751 Pubmed8242752 Pubmed8259214 Reactome Database ID Release 4369560 Reactome, http://www.reactome.org ReactomeREACT_202 p53 causes G1 arrest by inducing the expression of a cell cycle inhibitor, p21 (El-Deiry et al, 1993; Harper et al, 1993; Xiong et al, 1993). P21 binds and inactivates Cyclin-Cdk complexes that mediate G1/S progression, resulting in lack of phosphorylation of Rb, E2F sequestration and cell cycle arrest at the G1/S transition. Mice with a homozygous deletion of p21 gene are deficient in their ability to undergo a G1/S arrest in response to DNA damage (Deng et al, 1995).