The DYKDDDDK peptide (Flag) is used as a fusion tag for immunoaffinity chromatography, which is consisted of eight amino acids (Asp-Tyr-Lys-Asp-Asp-Asp-Asp-Lys) including an enterokinase-cleavage site. It allows elution under non-denaturing conditions. This epitope tag is a short hydrophilic, highly charged peptide. It is the most widely used epitope tag employed in structural and functional studies of proteins. 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+.