JAK2 and myeloproliferative neoplasm: Moreover, in these patients, the clinician and the pathologist have to look for (additional) signs of myeloproliferation, i.e. increased spleen size, leukocytosis with left shift, marked eosinophilia and/or basophilia, marked thrombocytosis, histologic features of an MPN, JAK2 V617F, or/and a markedly elevated serum tryptase level (>100 ng/ml) [7,12,25,30].