mutations in this gene are associated with erythrocytosis familial type 4, pulmonary hypertension and chronic mountain sickness. there is also evidence that certain variants of this gene provide protection for people living at high altitude such as in tibet. the effect is most profound among the tibetans living in the himalayas at an altitude of about 4,000 metres above sea level, the environment of which is intolerable to other human populations due to 40% less atmospheric oxygen.