The mutations in the JAK2, MPL and CALR genes are responsible for myeloproliferation by directly or indirectly deregulating JAK2 signaling resulting in two molecular entities: JAK2V617F mutated MPN where the same mutation can lead to three distinct phenotypes (ET, PV and PMF) depending on its allelic load and, MPL/CALR mutated MPN including ET and PMF. The gene discussed is JAK2; the disease is essential thrombocythemia.