These four patients were characterized by a relatively low mast cell burden with regard to mast cell infiltration in bone marrow histology and serum tryptase level but a very high KIT D816V EAB (representing disease burden of both SM and AHN) and a poor-prognostic molecular risk profile with ≥ 2 mutations in the S/A/R gene panel. The gene discussed is KIT; the disease is systemic mastocytosis.