Idiopathic erythrocytosis is a common condition in clinical hematology and is investigated in the first line with standard biological tests, including JAK2 mutation (exons 14 and 12), serum EPO, arterial blood gases (to rule out hypoxia or carbon monoxide poisoning) and venous blood gases (to detect HOAH or, rarely, bisphosphoglycerate mutase deficiency). The gene discussed is EPO; the disease is polycythemia.