As detailed in this review, ATM deficient cancers appear susceptible to ATR inhibition and therefore could be a subgroup with pronounced benefit.26 In the phase 1 study of berzosertib, a patient with metastatic colorectal cancer harboring an ATM mutation experienced a complete response.29 In the phase I study of elimusertib, all 4 patients who achieved a partial response had ATM aberrations.33 This is consistent with in vitro studies demonstrating synthetic lethality of ATR inhibitors in cell lines lacking ATM function. This evidence concerns the gene ATM and metastatic colorectal cancer.