The diagnosis of SM requires the presence of the major histological criterion, which consists of at least 15 multifocal dense MCs infiltrates in the BM or other extracutaneous organs, in addition to at least one minor criterion: (i) presence of atypical morphology in more than 25% of BM or extracutaneous MCs; (ii) serum tryptase (sT) >20 ng/mL; (iii) CD2 and/or CD25 positive MCs in BM or other extracutaneous organs; (iv) detection of KIT mutation at codon 816 in BM, blood or extracutaneous organs. Here, KIT is linked to systemic mastocytosis.