CHEK1 and cancer: Mechanistically, γH2AX is phosphorylated by phosphatidylinositol-3 kinase (PI3K)-like kinases, including ATM, ATM-Rad3-related kinase (ATR), ATM-related kinase (ATX) and other cell cycle checkpoint factors such as Chk1 and Chk2.29 Thus the presence of γH2AX foci is an excellent marker of DNA damage caused by cytotoxic agents or cancer treatments.29–31 Rajendran et al.,29 suggested that combination treatments could be very effective and produced synergistic actions when used to target DNA damage and interfere with DNA repair in cancer cells.