Myelofibrosis (MF) is one of the classic Philadelphia-negative myeloproliferative neoplasms (Ph-negative MPN) alongside essential thrombocythemia (ET) and polycythemia vera (PV). MF is a heterogeneous disease with a variable prognosis. The genetic underpinnings of MF and the other classic Ph-negative MPNs remained largely unknown until 2005, when four publications demonstrated the presence of the activating V617F mutations in the JAK2 non-receptor tyrosine kinase gene in patients with Ph-negative MPNs. While the discovery of the JAK2V617F mutation proved to be an important step forward in the understanding of the biology of MF and related MPNs, it didn't explain the whole complex biology of these disorders, and mutations in several other genes (e.g. MPL, SH2B3, CBL, TET2, ASXLJ, RUNXJ, SRSF2, IDHJ, IDH2 and EZH2) were recently found. Mutations in some genes (e.g. ASXLJ, SRSF2) also appear to be important prognostic markers, and are associated with an increased risk of transformation to acute myeloid leukemia and death.