ATR and colorectal cancer: Such tumour samples have been identified previously, including in the very first phase I dose escalation trial of an ATR inhibitor, which identified a colorectal cancer patient with 100% ATM loss by IHC, who showed a complete response (19+ months) to M660 monotherapy.43,44 Evaluating ATM functionality by IHC, using markers such as phospho-ATM and phospho-RAD50,34 in conjunction with total ATM could add further depth to ATM status assessments; however, this would likely require patients to be challenged with a DSB-inducing agent prior to biopsy to be most informative.