Streptavidin is a nonglycosylated, tetrameric protein, with each subunit able to bind a single molecule of the vitamin biotin. Streptavidin-biotin bond is the strongest known non-covalent interaction with Kd ~10-15 M. Because streptavidin lacks any carbohydrate modification and has a near-neutral pI, it has the advantage of much lower nonspecific binding than avidin. Streptavidin is broadly used in various applications such as immunoassays, histochemistry, FISH (Fluorescence In Situ Hybridization), flow cytometry, microarrays and blot analysis.