Streptavidin is a tetrameric protein composed of identic subunits. Each subunit binds one biotin molecule with a KD of ~1x10-15M. The preparation contains an N- and C-terminal shortened variant (core streptavidin) with improved properties concerning homogeneity, solubility, resistance towards proteolytic degradation and accessibility of the biotin binding pocket as compared to native streptavidin. The high affinity recognition of biotin and biotinylated molecules has made streptavidin one of