Systemic mastocytosis (SM) is a highly heterogeneous disease characterized by clonal (usually KIT mutants) proliferation and accumulation of abnormal mast cells in one or more extracutaneous organs, for example, the bone marrow, gastrointestinal tract, or liver, with a plethora of distinct presentations ranging from high mast cell burden to organ infiltration and failure. Here, KIT is linked to systemic mastocytosis.