Of note, a significant proportion of patients with indolent forms of WDSM fail to fulfill the current WHO 2008 diagnostic criteria for SM, despite they systematically show features of a systemic MC disease such as BM MC aggregates, aberrant CD30 expression on BM MCs, mutations involving any region of KIT and/or a clonal nature based on the HUMARA pattern of inactivation of the X chromosome. The gene discussed is KIT; the disease is systemic mastocytosis.