Infrequent occurrence of this unique JAK2 mutation has been reported in chronic myelomonocytic leukemia (CMML), atypical or unclassified myeloproliferative disorder (MPD), myelodysplastic syndrome (MDS), systemic mastocytosis (SM), and chronic neutrophilic leukemia (CNL) [10–16]. Here, JAK2 is linked to systemic mastocytosis.