Many Sherpa are highly regarded as elite mountaineers and experts in their local area. They were immeasurably valuable to early explorers of the Himalayan region, serving as guides at the extreme altitudes of the peaks and passes in the region, particularly for expeditions to climb Mount Everest. Today, the term is often used by foreigners to refer to almost any guide or climbing supporter hired for mountaineering expeditions in the Himalayas, regardless of their ethnicity. Because of this usage, the term has become a slang byword for a guide or mentor in other situations. Sherpas are renowned in the international climbing and mountaineering community for their hardiness, expertise, and experience at very high altitudes. It has been speculated that part of the Sherpas' climbing ability is the result of a genetic adaptation to living in high altitudes. Some of these adaptations include unique hemoglobin-binding capacity and doubled nitric oxide production.