ATR and cancer: 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.