CHEK1 and glioblastoma: Furthermore, the protein expression of RAD51 recombinase (catalyzes the core reactions of HR) and CHEK1 (a kinase central for the DDR signaling at the S and G2/M cell cycle checkpoints) was examined, and dose-related decreases of both protein levels were observed after MPT0B291 treatment in A172 GBM cells and TMZ-resistant A172 GBM cells (Fig. 2B), as well as in the patient-derived TMZ-resistant GBM cells (Fig. 2C).