Li and colleagues demonstrate that the cell-cycle checkpoint kinase CHK1 phosphorylates and inactivates FAM122A, an inhibitor of the phosphatase PP2A. Activated PP2A in turn dephosphorylates WEE1, prevents its ubiquitin-mediated proteolysis, increases WEE1 protein levels, and promotes the G2/M checkpoint. Loss of FAM122A expression is a mechanism of acquired CHK1 inhibitor resistance..