NTRK1 and gastrointestinal stromal tumor: However, GIST is now recognised as a separate tumour entity and is defined as a spindle cell, epithelioid or pleiomorphic mesenchymal tumour of the gastrointestinal tract that strongly expresses the KIT (CD 117) protein and may harbour mutations of the type III tyrosine kinase receptor gene (either KIT or PDGFRA) [10].