Gastrointestinal stromal tumor (GIST) is the preferred term for mesenchymal tumors specific for the gastrointestinal tract (60% in stomach, 30% small intestine, 10% elsewhere). GISTs include most tumors previously designated as leiomyoma, cellular leiomyoma, leiomyoblastoma, and leiomyosarcoma. However, in the esophagus, leiomyoma is the most common mesenchymal tumor. GISTs are composed of spindle (70%) or epithelioid (30%) cells, and 10%-30% are malignant showing intra-abdominal spread or liver metastases. They are immunohistochemically positive for c-kit (CD117), CD34, and sometimes for actin but are almost always negative for desmin and S100-protein. The malignant GISTs especially show activating mutations in the c-kit gene. GISTs and gastrointestinal autonomic nerve tumors (GANT) overlap. The cell of origin is not fully understood, but resemblance to the interstitial cells of Cajal, expression of some smooth muscle markers, and occurrence outside of the GI-tract suggest origin from multipotential cells that can differentiate into Cajal and smooth muscle cells.