CHEK1 and cancer: Examples of cancer therapeutics that induce or exacerbate RS include chemotherapeutics such as nucleoside analogs (5–7), hydroxyurea (HU) (which uncouples helicase from polymerase activity) (8), and inhibitors of key DDR kinases such as ataxia telangiectasia and Rad3-related protein (ATR) (9, 10), checkpoint kinase 1 (CHK1), and Wee1 (11, 12).