CHEK1 and glioblastoma: By following the formation of this damage after TMZ exposure and comparing its appearance with activation of the DNA damage response, we here show that the Chk1 phosphorylation previously linked only to TMZ-induced MMR-dependent DNA damage in TMZ-sensitive cells in fact occurs in all GBM cells examined long before the activation of Chk2, creation of MMR-dependent TMZ-induced DNA damage, and activation of the G2 checkpoint.