JAK2 and acquired polycythemia vera: Polycythemia vera (PV) is a myeloproliferative neoplasm characterized by elevated hematocrit (Hct), trilineage myeloproliferation in the bone marrow, and presence of Janus kinase (JAK)2 activating mutations.1 The estimated prevalence of PV in the United States is between 44 and 57 cases per 100 000 individuals, corresponding to ~150 000 to 200 000 US patients.2,3