To date, loss-of-function JAK2 mutations have not been reported while somatic and germline dominantly inherited gain-of-function JAK2 variants, particularly the recurrent p.Val617Phe, are well established causes of hematologic disorders (thrombocythemia-3, thrombocytopenia, polycythemia vera)37–39. Here, JAK2 is linked to acquired polycythemia vera.