GSTM1 and chronic mountain sickness: Relative to Tibetan and Ethiopian highlanders at comparable altitudes, Andeans show slightly higher average hemoglobin concentration ([Hb]) (1, 4) and many develop excessive erythrocytosis (EE; [Hb] ≥ 21 g/dL in men and ≥19 g/dL in women) and chronic mountain sickness (CMS, also called Monge’s disease).