CHK1 is activated by upstream kinases, such as ATR (Ataxia Telangiectasia and Rad3-related protein), in response to single-stranded DNA and stalled replication forks. Upon activation, CHK1 phosphorylates several downstream targets, including the CDC25 family of phosphatases, which in turn regulate CDKs (Cyclin-Dependent Kinases). This phosphorylation prevents cells from entering mitosis, thus providing time for DNA repair mechanisms to correct any damage.
