Tibetans have lived at very high altitudes for thousands of years, and they have 
a distinctive suite of physiological traits that enable them to tolerate 
environmental hypoxia. These phenotypes are clearly the result of adaptation to 
this environment, but their genetic basis remains unknown. We report genome-wide 
scans that reveal positive selection in several regions that contain genes whose 
products are likely involved in high-altitude adaptation. Positively selected 
haplotypes of EGLN1 and PPARA were significantly associated with the decreased 
hemoglobin phenotype that is unique to this highland population. Identification 
of these genes provides support for previously hypothesized mechanisms of 
high-altitude adaptation and illuminates the complexity of hypoxia-response 
pathways in humans.