KIT and gastrointestinal stromal tumor: Mutations in Kit’s juxta-membrane domain or tyrosine kinase domain 1 (TKD1) are common in GISTs,11, 23, 24 but rare in human neoplastic mast cell disorders.14 Neoplastic mast cells commonly have a mutation elsewhere, in the activation loop of Kit’s tyrosine kinase domain 2 (TKD2).14 GIST-type Kit may be insufficient for transforming mast cells.