The 2008 World Health Organization major diagnostic criteria for PV require erythrocytosis (i.e., elevated hemoglobin, hematocrit, red cell mass levels) or an activating mutation in JAK2; minor diagnostic criteria include bone marrow trilineage myeloproliferation, subnormal serum erythropoietin level, and endogenous erythroid colony growth [2]. The gene discussed is JAK2; the disease is acquired polycythemia vera.