mastocytosis (MONDO_0007950, a Monarch Disease Ontology entry) is a clonal myeloproliferative neoplasm characterized by the proliferation and accumulation of neoplastic mast cells in one or multiple organs or organ systems. It is a heterogeneous group of neoplasms, ranging from cutaneous proliferations which may regress spontaneously, to aggressive neoplasms associated with organ failure and short survival. Also known as: Mast cell disease, mast cell hyperplasia.