S期
Between G1 and S phase, three DNA damage checkpoints occur to ensure proper growth and synthesis of DNA prior to cell division. Damaged DNA during G1, before entry into S phase, and during S phase result in the expression of ATM/R protein. ATM/R protein then stabilizes and activates transcription factor p53 so that it can bind to upstream regions of genes, inducing the expression of proteins including p21CIP. p21CIP binds to and inhibits any cyclin-cdk present in the cell cycle, halting the cycle until DNA damage can be corrected.