Functional relationship between Claspin and Rad17

► This is the first study to characterize Claspin gene knockout cells using DT40 cells. ► Claspin plays a critical role in DNA replication in the absence of exogenous stress. ► This is the first study to reveal the functional relationship between Claspin and Rad17. ► RAD17− cells observed a greater defect in MMS-induced checkpoint activation than Claspin−/− cells. ► Knocking out RAD17 gene showed almost no additive effects in Claspin−/− cells.