The Nano-tag is a new streptavidin-binding peptide for both the purification and the detection of Nano-tagged proteins. This peptide possesses nanomolar-affinity for streptavidin and therefore is termed Nano-tag. The nano-tags have two types, Nano-tag15(MDVEAWLGARVPLVET) and Nano-tag9(MDVEAWLGAR, which bind to streptavidin with dissociation constants of 4 nM and 17 nM, respectively.