The main entities of MPNs, Polycythemia Vera (PV), Essential Thrombocythemia (ET), and Primary Myelofibrosis (PMF) are frequently reveal a genetic mutation, JAK2V617F which involves a change in the Janus kinase 2 (JAK2) gene, c.1849G>T, leading to the constitutive activation of the Janus kinase-signal transducer and activator of transcription (JAK/STAT) signaling pathway. The gene discussed is JAK2; the disease is primary myelofibrosis.