Originally described by Dameshek in 1951, myeloproliferative disorders are today classified as myeloproliferative Neoplasms (MPNs) in WHO's Classification of Tumors of Hematopoietic and Lymphoid Tissues. The term includes a range of conditions, [ie, BCR-ABL-positive chronic myelogenous leukemia (CML), chronic neutrophilic leukemia (CNL), polycythemia vera (PV), primary myelofibrosis (PMF), essential thromobocythemia (ET), chronic eosinophilic leukemia not otherwise specified (CEL-NOS), mastocytosis, and unclassifiable myeloproliferative neoplasm]. In the specific case of CML, a better understanding of the pathogenesis and pathophysiology of the disease has led to a targeted therapy. The presence of chromosome Philadelphia, t(9;22)(q34;11) results in the oncogene BCR-ABL, which characterizes the disease; this molecular rearrangement gives rise to a tyrosine-kinase, which in turn triggers the proliferation of the myeloid line through the activation of the signaling pathways downstream. Tyrosine-kinase inhibitors (TKIs) have altered the therapy and monitoring of CML patients and improved both their prognosis and quality of life. In 2005, various groups of investigators described a new point mutation of the gene JAK2 associated to MPNs. Although the presence of this mutation has led to a modification in the diagnostic criteria of these conditions, the impact of the use of JAK2 inhibitors on the prognosis and course of the disease continues to be controversial.