Gastrointestinal stromal tumours. Endoscopic images of submucosal gastrointestinal stromal tumours (GISTs) in a patient's stomach. GISTs are rare tumours that develop from mesenchymal tissue (muscle, connective tissues and bone) of the gastrointestinal (GI) tract. Symptoms include trouble swallowing (dysphagia), gastrointestinal haemorrhage or secondary tumours. Most occur in the stomach, where they have a lower malignant (cancerous) potential than tumours found elsewhere in the GI tract.