Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal neoplasms of the gastrointestinal tract. GISTs arise in the smooth muscle pacemaker interstitial cell of Cajal, or similar cells.[2] They are defined as tumors whose behavior is driven by mutations in the KIT gene (85%),[2] PDGFRA gene (10%),[2] or BRAF kinase (rare).[2] 95% of GISTs stain positively for KIT (CD117).[2][3] Most (66%) occur in the stomach and gastric GISTs have a lower malignant potential than tumors found elsewhere in the GI tract.[3]

Reference
Wiki: Gastrointestinal stromal tumor