 This study is is the first step of a full study named CHEMOGENE because it explores the genetic determinant of an alteration of the chemoreflex. This reflex determines hyperventilation when the pressure of oxygen falls in the blood. This happens when subjects travel to high-altitude where oxygen levels diminish in the atmosphere. Subjects with such an altered chemoreflex are intolerant to altitude and develop pulmonary or cerebral edema associated with a severe headache. In this study we compare subjects tolerant to high altitude 8000 metersto subjects intolerant to altitude. The chemoreflex is measured i.e. the hyperventilation associated with hypoxia and all subjects are scanned for the genes implicated in the mitochondrial respiratory chain. The idea is that subjects with an impaired oxygen sensing will exhibit an altered chemoreflex and will be intolerant to high-altitude. 