Fibronectin exists as a protein dimer, consisting of two nearly identical polypeptide chains linked by a pair of C-terminal disulfide bonds. Each fibronectin subunit has a molecular weight of 230–250 kDa and contains three types of modules: type I, II, and III. All three modules are composed of two anti-parallel β-sheets resulting in a Beta-sandwich; however, type I and type II are stabilized by intra-chain disulfide bonds, while type III modules do not contain any disulfide bonds. The absence of disulfide bonds in type III modules allows them to partially unfold under applied force.