Gastrointestinal stromal tumour, primary site

**Definition:** This is the most common mesenchymal tumour that arises in the gastrointestinal tract. It is generally immunohistochemically positive for CD117 (KIT), phenotypically paralleling Cajal-cell differentiation, and most examples contain KIT- or PDGFRA-activating mutations. It is most frequent in the stomach and to a lesser degree in the small intestine. The prognosis depends on the tumour size and the mitotic activity.