Polycythaemia vera (PV) is a Janus kinase 2 (JAK2)–mutated myeloproliferative neoplasm (MPN) characterised by clonal erythrocytosis, with increased risk of thrombosis and progression to myelofibrosis or acute myeloid leukaemia [1]. The gene discussed is JAK2; the disease is myeloproliferative disorder.