Although ATM mutations are present in 5% of primary gastric cancers, loss of ATM protein expression has been reported to occur in 17% of the cases (6, 17–19), while truncating mutations in ARID1A are present in approximately 20% of gastric cancer (6, 18, 19), which supports the case for the clinical assessment of ATRi in gastric cancer in appropriately stratified populations. The gene discussed is ARID1A; the disease is gastric cancer.