It has recently been reported that ATM-null mouse thymocytes have mitochondrial DNA damage and enhanced levels of reactive oxygen species (ROS)69 and a recent report has shown that olaparib induces ROS in bladder cancer cells.43 However, we did not see evidence of elevated ROS in either ATM-deficient cells or A549 control cells treated with olaparib, suggesting that ROS do not contribute to olaparib induced toxicity in A549 cells. This evidence concerns the gene ATM and urinary bladder cancer.