SMAD4 is a tumor suppressor gene and it encodes an intracellular mediator in the transforming growth factor β (TGF β) signal transduction pathway. The gene is inactivated in 40% of human gastric cancers by loss of heterozygosity, promoter hypermethylation, and somatic mutation. Prior sequencing studies have shown that SMAD4 is mutated in up to 8% of gastric cancers. SMAD4 p.R361H mutation occurs within the MH2 domain which is the SMAD-SMAD interaction and transcription activation domain of the protein. The loss of SMAD4, especially loss of nuclear SMAD4 expression, is involved in gastric cancer progression. Mutations at codon 361 have been previously reported in various tumor types. The exact clinical significance of this mutation in gastric cancers needs to be fully elucidated.