CHEK1 and non-small cell lung carcinoma: Treatment with 0.2 mm HU increased activated pCHK1 Ser317 and CHK1 activity (demonstrated by elevated RRM2 levels) in panels of melanoma TSs and NSCLC cell lines, but had little effect on pRPA2 S4/8 and γH2AX levels (Fig. 4A,B).