JAK2 is a non receptor tyrosine kinase that plays a major role in myeloid development by transducing signals from diverse cytokines and growth factor receptors, a mutation in this Janus kinase interferes with the negative regulation of JAK2 and could account for the hypersensitivity of myeloid cells from MPD patients to growth factors.JAK2 V617F mutation may play a very important role not only in the development of MPD but also in phenotypic presentation of the disease, as well as it may be useful for the diagnosis, management and follow up of the disease as the mutation increases on relapses and decreases denoting response to treatment hence it may be also used as a predictive test for the treatment outcome in CMPDs patients.