CHEK1 and cancer: An acute pHe titration of the medium bathing native HCT116 cancer cells confirmed a pH-dependent elevation in phospho-CHK1 levels; phospho-CHK2 was also increased upon acute exposure to pHe 6.5 but not at lower pHe probably because of the development of cytotoxicity at pHe < 6.0 (Fig. 4B).