Monarch Disease Ontology id MONDO_0020334 (mast cell leukemia) is a malignant form of systemic mastocytosis (SM) characterized, most of the time, by the presence of circulating mast cells. Also known as: Mast cell leukaemia, Mast cell leukemia, aleukemic mast cell leukaemia, aleukemic mast cell leukemia, mast-cell leukaemia, mast-cell leukemia.