The discovery of KIT gene mutation in gastrointestinal stromal tumor (GIST) has provided a paradigm shift in the classification, diagnosis, and molecular-targeted therapy of gastrointestinal mesenchymal tumors. According to a recent concept, GIST is considered a spindle or epithelioid cell neoplasm which basically expresses KIT protein and has KIT or platelet-derived growth factor receptor-alpha (PDGFRA) gene mutation. Exceptional cases are immunohistochemically negative or weakly positive for KIT (often with PDGFRA mutation), and minor subset has another gene alteration such as succinate dehydrogenase (SDH), RAS, NF1, or BRAF. There are growing evidences of phenotype-genotype correlations in GIST. Risk stratification based on mitotic counts, tumor size, and rupture is useful for the prognostication and management of patients with GIST. GISTs should be distinguished from various types of neoplasms such as leiomyoma, schwannoma, and inflammatory myofibroblastic tumor, although leiomyosarcoma of the gastrointestinal tract has become a very rare entity in the "KIT" era. Both histopathological procedures and molecular investigations are important for the evolution of diagnosis and treatment of GIST and mimics.
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