In a cohort of 1638 PV patients enrolled in the European Collaborative Low-dose Aspirin Polycythemia Vera (ECLAP) study5 and in a recent large observational study, including 1545 patients with JAK2-mutated PV6, the incidence was 1.2 and 0.9 per 100 patient-years, respectively. The gene discussed is JAK2; the disease is acquired polycythemia vera.