Chronic myeloproliferative neoplasms (MPN) comprise a spectrum of clonal 
neoplastic disorders characterized by overproduction of terminally 
differentiated cells of the myeloid lineage. A common genetic basis for the 
BCR-ABL-negative MPN disorders was elucidated in 2005 with the identification of 
the JAK2V617F mutation in the majority of MPN patients. The discovery of 
JAK2V617F had a dramatic impact on the diagnosis and treatment of MPN. Testing 
for JAK2 mutations is now included in the World Health Organization (WHO) 
criteria for the diagnosis of MPN, and in 2011 the oral JAK2 kinase inhibitor 
ruxolitinib became the first Food and Drug Administration (FDA)-approved drug 
for the treatment of myelofibrosis. The drug is now also approved in Europe and 
Canada.