JAK2 and polycythemia: Both acquired and familial erythrocytosis (FE), except relative erythrocytosis, are examined by subgrouping as primary and secondary.[1] Primary-acquired erythrocytosis involves polycythemia vera (PV), which is characterized by the presence of JAK2 mutations, although these markers are not found in about 1% of patients with PV.[2–4] Secondary-acquired erythrocytosis, however, occurs when erythropoietin (EPO) drives the production of red cells.