Major structural protein of tissues such as aorta and nuchal ligament, which must expand rapidly and recover completely. The polymeric elastin chains are cross-linked together into an extensible 3D network. Extracellular matrix of elastic fibers. Additional isoforms seem to exist. Elastin is formed through the cross-linking of its soluble precursor tropoelastin. Cross-linking is initiated through the action of lysyl oxidase on exposed lysines to form allysine. Subsequent spontaneous condensation reactions with other allysine or unmodified lysine residues result in various bi-, tri-, and tetrafunctional cross-links. The most abundant cross-links in mature elastin fibers are lysinonorleucine, allysine aldol, desmosine, and isodesmosine. Hydroxylated on proline residues. Hydroxylation on proline residues within the sequence motif, GXPG, is most likely to be 4-hydroxy as this fits the requirement for 4-hydroxylation in vertebrates. Belongs to the elastin family.