Distinguishing a leiomyosarcoma from the most commonly encountered mesenchymal gastrointestinal tumor (i.e., the gastrointestinal stromal tumor, GIST [29], characterized by the presence of activating mutations in KIT or PDGFRA, and expression of CD 117 and/or CD34) is highly important. The gene discussed is KIT; the disease is leiomyosarcoma.