Protein tags are protein or peptide sequences located either on the C- or N- terminal of the target protein, which facilitates one or several of the following characteristics: solubility, detection, purification, localization and expression. The DYKDDDDK(FLAG) peptide has been used extensively as a general tag in expression vectors. This peptide can be expressed and detected with the protein of interest as an amino-terminal or carboxy-terminal fusion. N-terminal FLAG vectors provide an Ek cleavage site for removal of the fusion tag. The FLAG peptide is likely to be located on the surface of a fusion protein because of its hydrophilic nature. As a result, the FLAG peptide is more likely to be accessible to antibodies.