Furthermore, our results using radiation cytogenetics reinforce the concept that ATM, ATR, and Chk1 represent attractive anticancer drug targets in radiation oncology since (i) resistance to genotoxic therapies has been associated with an increased DDR signaling, and (ii) many cancers have defects in certain components of the DDR, rendering them highly dependent on the remaining DDR pathways for survival. This evidence concerns the gene CHEK1 and cancer.