Rarely erythrocytosis is a consequence of single gene variant (monogenic disorder), that can be acquired (JAK2, indicative for polycythaemia vera, PV) or inherited (EPOR, VHL, EGLN1, EPAS1, EPO, HBB, HBA, BPGM, indicative for congenital erythrocytosis, ECYT) [1,2]. The gene discussed is EPOR; the disease is polycythemia.