EPAS1 and chronic mountain sickness: The low prevalence among Tibetans of diseases associated with elevated Hb concentration, such as chronic mountain sickness [69], and a signal of selective sweep in the EPAS1 gene [33, 34] have led to the hypothesis that unelevated Hb is adaptive in Tibetan highlanders [70]; this hypothesis was also substantiated by the correlation between low Hb and better reproductive outcomes in our Tibetan sample [48].