Acts as an inhibitor of serine/threonine-protein phosphatase 2A (PP2A) activity (PubMed:27588481, PubMed:33108758). Potentiates ubiquitin-mediated proteasomal degradation of serine/threonine-protein phosphatase 2A catalytic subunit alpha (PPP2CA) (PubMed:27588481). Inhibits PP2A-mediated dephosphorylation of WEE1, promoting ubiquitin-mediated proteolysis of WEE1, thereby releasing G2/M checkpoint (PubMed:33108758). Interacts with PPP2CA and PPP2R1A (PubMed:27588481). Interacts with PPP2R2A (PubMed:27588481, PubMed:33108758). The CHEK1-mediated Ser-37 phosphorylated form interacts with 14-3-3 proteins (PubMed:33108758). The CHEK1-mediated Ser-37 phosphorylated form is sequestered by 14-3-3 proteins in the cytoplasm and fails to translocate to the nucleus, where it otherwise inhibits serine/threonine-protein phosphatase 2A. CHEK1-mediated phosphorylation at Ser-37 negatively regulates its ability to inhibit serine/threonine-protein phosphatase 2A (PP2A) activity. Phosphorylation leads to its release from the PP2A complex and its sequestration by 14-3-3 proteins in the cytoplasm resulting in its inability to translocate to the nucleus, where it otherwise inhibits PP2A. Belongs to the FAM122 family.