The absence of mutations in other DDR-related genes such as ataxia-telangiectasia mutated (ATM), BRCA1, mutL homolog 3 (hMLH3), and Nijmegen breakage syndrome protein 1 (NBS1) suggested a selective vulnerability of the ATR-CHK1 axis in MSI-high gastric cancers. Here, BRCA1 is linked to gastric cancer.