The prominent mass seen here arose in the muscular wall of the stomach and grew exophytically toward the lumen, retaining a covering of mucosa except in the central ulcerated area. This is a gastrointestinal stromal tumor (GIST). These lesions used to be classified as smooth muscle tumors, but are now thought to arise from the interstitial cell of Cajal, a cell that forms part of the myenteric plexus.