Germline mutations in CDH1, the gene coding for the E-cadherin adhesion protein, are known to cause hereditary diffuse gastric cancer. We identified a new truncating germline mutation (p.Asp538Thrfs*19) in exon 11 of the CDH1 gene in a 41-year-old male with a diffuse gastric cancer. Although he had no parental history of gastric cancer, the co-segregation study in the family detected the same mutation in his healthy 31-year-old brother. The mutation affects one of the extracellular repeat (CAD repeats) domains which is essential for the homophilic binding specificity that directs "E-cadherin" to bind with itself each others. In this case, immunohistochemical analysis showed no expression of E-cadherin in the tumor sample and was a useful prescreening tool to genetic testing. This finding was associated with a poor response to trastuzumab-based treatment.