MCAS can be classified into: a) primary MCAS, also known as monoclonal MCAS (=MMAS) where clonal KIT-mutated MC are found; b) secondary MCAS where an IgE-dependent allergy (most cases) or another reactive inflammatory disease process is present (and is considered to be the causative etiology); and c) idiopathic MCAS where neither clonal MC nor an IgE-dependent allergy or another underlying condition/disease can be documented [66,67,68,69]. This evidence concerns the gene KIT and allergic disease.