JAK2 is part of the network that allows cells to communicate with their environment. Certain hormones in the body called cytokines control blood formation by signaling through JAK2. The JAK2 gene mutation is found in nearly all people who have polycythemia vera (PV) and 30-50% of people who have essential thrombocythemia (ET) or myelofibrosis (MF). The JAK2 mutations in MPNs lead to excess blood cell formation. This occurs because the mutated JAK2 sends signals even when it isn’t supposed to do so and this leads to excess blood cell production.  There are now drugs that help block signaling through JAK2 (JAK2-inhibitors).