The FNDC5 gene encodes a protein that is secreted from muscle cells during exercise. This protein is called Fibronectin type III domain-containing protein 5, and it is thought to play a role in fat development. The protein is actually a prohormone that undergoes post-translational processing to produce a protein called Irisin. Because of its release by muscle cells in response to muscular contraction, Irisin is classed as a myokine. There is evidence to show that Irisin may be involved in the development of brown fat, weight loss, and protection against diabetes, at least in mice.
It has been postulated that Irisin could be used for therapeutic intervention for metabolic disease and other disorders that are improved with exercise. Mutations in this gene cause type 2 diabetes.