Streptavidin is a 52,800 dalton tetrameric protein purified from the bacterium Streptomyces avidinii. It has an extraordinarily high affinity for biotin (also known as vitamin B7), the dissociation constant (kDa) of the biotin-streptavidin complex is on the order of 10-14 mol/L. Streptavidin is used extensively in molecular biology and bionanotechnology due to the streptavidin-biotin complex's resistance to organic solvents, denaturants , detergents, proteolytic enzymes, and extremes of temperature and pH.