Disease progression and prognosis in PMF are usually associated with worsening of anemia, increase of circulating blasts, and, more recently, with the presence, in addition to the “classical” driver mutations, of JAK2, MPL, and CALR genes, as well as of cytogenetic and molecular abnormalities that have been incorporated into new genetically based prognostic scoring systems. The gene discussed is JAK2; the disease is anemia.