The introduction of imatinib mesylate (Glivec®), a small-molecule tyrosine kinase inhibitor active against BCR-ABL, KIT, and PDGFR, changed dramatically the prognosis of unresectable or metastatic/advanced GIST patients, achieving a disease response in more than 50% of treated cases and stabilisation in almost 30%. The gene discussed is KIT; the disease is gastrointestinal stromal tumor.