Streptavidin, with a molecular weight of about 60 kDa, is a bacterial protein derived from Streptomyces avidinii that binds biotin with high specificity and an affinity constant (Kd) of 10-15M. As a tetrameric protein, it can bind four biotin molecules simultaneously, a function that is highly similar to that of the egg-white derived avidin. However, unlike avidin, streptavidin is a non-glycosylated protein and is largely uncharged, whereas avidin is basic at neutral pH, and the non-specific binding of streptavidin is much lower than that of avidin in comparison, so that the non-specific background of streptavidin is significantly lower for detection. Currently, biotin assays are usually performed with streptavidin instead of avidin. This recombinant expressed streptavidin of Escherichia coli has a purity of ≥ 95% and is suitable for coupling enzyme preparations (e.g. HRP, ALP), fluorescein (e.g. FITC, PE, Alexa Fluor series probes, Dylight series probes), magnetic beads, etc., resulting in the detection of biotin-labeled antibodies, nucleic acids, proteins or other biotin-labeled molecules, and this streptavidin- biotin signal amplification/detection system is commonly used for ELISA, immunohistochemistry, Southern Blot or other DNA/RNA analysis; and purification of biotin-labeled molecules, etc.