Collagen is a fibrous protein found in the extracellular matrix and connective tissue. Type I collagen is the most common form of collagen prevalent in bones, tendons and skin. It consists of three intertwined coiled subunits: two α1 (I) chains and one α2 (I) chain. Each chain contains precisely 1050 amino acids wound tightly around one another in a characteristic right-handed triple helix. The triple-helical structure of collagen arises from its unusual abundance of three amino acids: glycine, proline, and hydroxyproline. These amino acids in collagen appear in a characteristic repeating motif Gly-X-Y, where X is usually proline and Y is usually hydroxyproline.