CHEK1 and neoplasm: However, the mechanism by which CHK1 is activated does not appear to influence either tumour or normal tissue sensitivity to the CHK1i, as 0.1–0.2 mm HU effectively sensitised tumour cells to the CHK1i, whereas normal fibroblasts retained similar levels of proliferative potential after CHK1i treatment with either 0.1 or 2 mm HU.