All Thoroughbreds have the particularity of having rather fine and sparse hair and a very thin skin that reveals their blood vessels and musculature with each effort. The development of their muscular tissues is at the origin of their aptitude for speed. A mutation in the gene that codes for myostatin, the protein regulating this development, would have appeared in a Thoroughbred mare in the second half of the 19th century. She later transmitted this famous speed gene to her offspring.