Nevertheless, as safety precautions, we will (i) apply a 30-min lead-in phase, where oxygen levels are reduced slowly from 16 to 12% to allow for physiological adjustment; (ii) exclude individuals with risk factors for altitude sickness; (iii) monitor potential symptoms of altitude sickness at a daily basis with the Environmental Symptoms Questionnaire (ESQ) related to cerebral function [83]; and (iv) collaborate with medical doctors at the Frederiksberg Hospital (LVK, MBJ, CBF) as in our previous EPO intervention studies. The gene discussed is EPO; the disease is altitude sickness.