CHEK1 and glioblastoma: Treatment of U251MG and Hs683 glioblastoma cells with two different lomustine concentrations for 24 h led to increased phosphorylation of several DNA-damage response markers, such as BRCA-1 (breast cancer 1 gene), ATM (ataxia-telangiectasia-mutated gene), ATR (ataxia-telangiectasia and Rad3 related), Chk-1 (checkpoint kinase 1), and H2A.X (H2A histone family member X), but we did not observe differences in phosphorylation between control and DIRAS-1 or -2 transfected cells (Figure 6A).