A slight majority of gastric cancer cases occurred among noncarriers of pathogenic MMR mutations (23 cases, 59%); however, 16 cases (41%) were detected among members of families with pathogenic mutations (9 in MSH2 and 7 in MLH1); these subjects had gastric cancer with a low age at onset (mean, 49 years old). The gene discussed is MSH2; the disease is gastric cancer.