Streptavidin is a ∼60 kDa homotetramer. Streptavidin binds four molecules of biotin with the highest affinity. The binding affinity of biotin to streptavidin is one of the highest reported for a non-covalent interaction to date, with a KD ∼ 0.01 pM. Streptavidin has an immunosuppressive role.