Polycythemia vera is a myeloproliferative neoplasm thought to be caused by an activating mutation in the JAK2. A normal JAK2 is a key regulator of hematopoiesis. Mutation in the JAK2 causes an overproduction of erythroid, myeloid and megakaryotic progenitor cells. This illustration shows the JAK/STAT pathway oversignals without cytokines binding to the receptor. This leads to an overproduction of dimerized STAT proteins which control downstream transcription causing erythrocytosis.