BACKGROUND: In approximately 10% of all gastric cancer (GC) cases, a heritable cause is suspected. A subset of these cases have a causative germlineCDH1mutation; however, in most cases the cause remains unknown. Our objective was to assess to what extent these remaining cases may be explained by germline mutations in the novel candidate GC predisposing genesCTNNA1,MAP3K6orMYD88. METHODS: We sequenced a large cohort of unexplained young and/or familial patients with GC (n=286) without aCDH1germline mutation for germline variants affectingCTNNA1,MAP3K6andMYD88using a targeted next-generation sequencing approach based on single-molecule molecular inversion probes. RESULTS: Predicted deleterious germline variants were not encountered inMYD88, but recurrently observed inCTNNA1(n=2) andMAP3K6(n=3) in our cohort of patients with GC. In contrast to deleterious variants inCTNNA1, deleterious variants inMAP3K6also occur frequently in the general population. CONCLUSIONS: Based on our resultsMAP3K6should no longer be considered a GC predisposition gene, whereas deleteriousCTNNA1variants are confirmed as an infrequent cause of GC susceptibility. BiallelicMYD88germline mutations are at most a very rare cause of GC susceptibility as no additional cases were identified.