CCNA2 and cancer: ATR subsequently activates Chk1 which signals downstream to halt cell cycle progression via inhibition of CDK2 (cyclin dependent kinase 2)/cyclin A in the S phase or in G2/M via inhibition of CDK1 paired with either cyclin B or cyclin A. As discussed above, complete loss of ATR function is not tolerated in replicating cells in contrast to ATM and DNA-PK which may be lost in human cancers.