Elastin is a protein in connective tissue that is elastic and allows many tissues in the body to resume their shape after stretching or contracting. Elastin helps skin to return to its original position when it is poked or pinched. Elastin is also an important load-bearing tissue in the bodies of mammals and used in places where mechanical energy is required to be stored. Elastin is particularly abundant in large elastic blood vessels such as the aorta. It is also very important in the lungs, elastic ligaments, the skin, the bladder, and elastic cartilage. Elastin is composed largely of glycine, proline, and other hydrophobic residues and contains multiple lysine-derived crosslinks, such as desmosines, which link the individual polypeptide chains into a rubberlike network. The hydrophobic regions of the chains, between the crosslinks, are highly mobile. The hydrophobic and crosslinking domains are coded by separate, small (27-114 bp) exons that are separated by large introns. The initial translation product is a 72kD polypeptide, designated tropoelastin. Elastin is made by linking many soluble tropoelastin protein molecules, in a reaction catalyzed by lysyl oxidase, to make a massive insoluble, durable cross-linked array.