For example, positive selection has been found on prolyl hydroxylase 2 (PHD2) coding gene EGLN1, where adaptive amino acid changing mutations increased HIF-2α degradation to reduce the hemoglobin (Hb) concentration in Tibetans, protecting them from polycythemia, a condition considered as a blunted physiological response at high altitude6. Here, EGLN1 is linked to polycythemia.