Tissues such as skin, lungs, and blood vessels must be strong as well as elastic to work properly. Tissues owe their elastic properties to the elastic fibers present in their extracellular matrix. Each of these fibers is composed of tropoelastin aggregates assembled on the microfibrils. Tropoelastins are the precursors of elastin molecules, made up of alternating hydrophobic and cross-linking domains. The hydrophobic motifs, which cover more than 80 percent of the total protein, allow elastin to self-associate, stretch, and recoil when needed. The cross-linking domains allow cross-links to form between neighboring tropoelastin molecules. Together, these structural features allow fibres to stretch more than fivefold compared to a rubber band of a similar cross-section. As elastin fibers mature, the elastin aggregates form an amorphous core surrounded by microfibrils. These elastin structural features confer remarkable tissue elasticity.