Congenital erythrocytosis can be either associated with reduced P50 (partial pressure of oxygen at which 50% of hemoglobin is saturated with oxygen), as in high-oxygen-affinity hemoglobinopathy, or with normal P50, as in Von Hippel–Lindau syndrome (VHL), Egl-9 family hypoxia inducible factor 1 (EGLN1), also called prolyl hydroxylase domain 2 (PHD2), endothelial PAS domain-containing protein 1 (EPAS1), also known as hypoxia-inducible factor-2alpha (HIF-2alpha), or erythropoietin receptor (EPOR) mutated patients. This evidence concerns the gene EPOR and polycythemia.