KIT and leiomyosarcoma: Most GISTs are associated with activating mutations in either KIT or PDGFRA. 6,7,8 Before the use of KIT (CD117) immunostaining as a GIST marker in the early 2000s,9 most GISTs were diagnosed as smooth muscle tumors (eg, leiomyosarcomas), neurofibromas, or schwannomas.3,10,11