One of the mechanisms by which IR exerts its effects on cancer cells is the activation of the ataxia telangiectasia and Rad3-related (ATR)–checkpoint kinase 1 (Chk1) pathway by DNA single-strand breaks or the activation of the ataxia telangiectasia mutated (ATM)–Chk2 pathway by DNA double-strand breaks. This evidence concerns the gene CHEK1 and cancer.