Biotin variants, streptavidin variants and their use

An object of the present invention is to provide a streptavidin mutant having reduced affinity for the native biotin, providing biotin variants with high affinity for the low affinity of streptavidin mutant further for this natural biotin it is. According to the present invention, including (a) a streptavidin mutant affinity was reduced with natural biotin or biocytin, and (b) biotin variants with high affinity for the streptavidin mutant, treatment or reagent kit for the diagnosis is provided.