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]. This evidence concerns the gene JAK2 and myeloproliferative neoplasm.