Polycythemia vera (PV) is a myeloproliferative neoplasm (MPN) marked by acquisition of an activating mutation of Janus kinase-2 (JAK2) [1, 2], which leads to erythrocytosis and frequently leukocytosis and thrombocytosis [3], and is associated with a high symptom burden [4, 5] and disease progression [6]. Here, JAK2 is linked to polycythemia.