Apart from well-known genetic abnormalities, several studies have reported variations in protein expression in Philadelphia-negative myeloproliferative neoplasm (MPN) patients that could contribute toward their clinical phenotype. In this context, a quantitative mass spectrometry proteomics protocol was used to identify differences in the granulocyte proteome with the goal to characterize the pathogenic role of aberrant protein expression in MPNs. LC/MS-MS (LTQ Orbitrap) coupled to iTRAQ labeling showed significant and quantitative differences in protein content among various MPN subtypes polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF), and according to the genetic status of ( presence and allele burden). A number of differentially expressed proteins were identified, with the most frequent being members of the RAS GTPase family and oxidative stress regulatory proteins. Subsequent analysis found that calreticulin (CALR), known to be involved in calcium homeostasis and apoptotic signaling, was overexpressed in granulocytes compared with wild type and independently of the allele burden. Finally, it was demonstrated, in a Ba/F3 cell model, that increased calreticulin expression was directly linked to and could be regulated by JAK2 kinase inhibitors. In conclusion, these results reveal proteome alterations in MPN granulocytes depending on the phenotype and genotype of patients, highlighting new oncogenic mechanisms associated with mutations and overexpression of calreticulin. .