Rheumatoid arthritis (RA) is a chronic autoimmune disease characterized by synovial inflammation and joint destruction, whereas polycythemia vera (PV) is a myeloproliferative neoplasm driven by the Janus kinase (JAK)2 V617F mutation, resulting in erythrocytosis and increased thromboembolic risk. The gene discussed is JAK2; the disease is myeloproliferative disorder.