PDGFRA and schwannoma: Immunohistochemistry also helps rule out other tumors in the differential diagnosis, since they typically lack desmin and S-100 expression, which serve as markers of smooth muscle and neural differentiation, respectively, thus aiding in the differentiation of GISTs from leiomyomas and schwannomas [5-6]. Some GISTs with epithelioid morphology instead carry PDGFRA mutations, including the imatinib-resistant exon 18 D842V variant [7].