KIT and systemic mastocytosis: More than 90% of adult patients with indolent systemic mastocytosis (ISM) have an activating mutation in KIT, the receptor for stem cell factor, which results in ligand-independent activation of KIT. Life-threatening forms of mastocytosis, including aggressive mastocytosis, systemic mastocytosis associated with a hematologic neoplasm, and MC leukemia, are treated with cytoreductive therapy.