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Streptavidin is a protein that has similar binding properties to egg white avidin. It is isolated from streptomyces avidinii. Streptavidin has a molecular weight of 60 kDa and has 4 subunits. Each subunit can bind one molecule of biotin. Biotin is a water- soluble vitamin. Streptavidin has an extremely high binding affinity (Kd=10-15) for biotin. Streptavidin conjugated to horseradish peroxidase has proven useful in the detection of antigens coupled with biotinylated secondary antibodies. There are several advantages when using a streptavidin-conjugates versus an ABC complex.In contrast to avidin, streptavidin is not glycosylated and is therefore uncharged at neutral pH (6.5 versus 10).This lowers nonspecific background staining. Streptavidin also lacks carbohydrate side chains that may be another cause of non-specific background. Another key advantage of streptavidin is the significant increase in sensitivity (probably due to less steric hindrance),thus facilitating an increase in overall binding capacity. Streptavidin-HRP is much more stable than the ABC complex. The ABC complex must be freshly made 30 minutes prior to use and is stable only for a few days. In contrast, the Streptavidin-HRP reagent is very stable, color-coded, and can be stored for up to 1-2 years. Biocare’s Streptavidin-HRP comes in a ready-to-use format, thus saving time and potential mistakes.