DOG1 is expressed ubiquitously in gastrointestinal stromal tumors irrespective of c-kit or PDGFR alpha mutation status. Reactivity for DOG1 may aid in the diagnosis of gastrointestinal stromal tumors, including PDGFRA mutants that fail to express c-kit antigen, and lead to appropriate treatment with imatinib mesylate, an inhibitor of the c-kit tyrosine kinase.