Purity >95%. A fusion tag;called DYKDDDDK and consisting of eight amino acids (Asp-Tyr-Lys-Asp-Asp-Asp-Asp-Lys) including an enterokinase-cleavage site;was specifically designed for immunoaffinity chromatography. It allows elution under non-denaturing conditions. Several antibodies against this peptide have been developed. One antibody;denoted as M1;binds the peptide in the presence of bivalent metal cations;preferably Ca2+. Elution is effected by chelating agents. Another strategy is competitive elution with excess of free DYKDDDDK peptide.