KIT and systemic mastocytosis: In more than 80% of adult patients with systemic mastocytosis, KIT mutations are found in the kinase domain (KD) of KIT (exon 17), commonly in position 816 (D816V), while in childhood mastocytosis D816V has a lower incidence, and a number of mutations occur in the extracellular domain of KIT (exons 8 and 9).