The DYKDDDDK tag, also known as the FLAG tag, is a widely used octapeptide epitope containing the amino acid sequence of the same name. It has a molecular weight of 1012 Da and this small size makes it unlikely to interfere with protein structure and function. It can be incorporated at the N- or C- termini of fusion proteins and may also be used in tandem, such as with 3xFLAG. Due to the popularity of the DYKDDDDK epitope, many antibodies are available for the immunological investigations of DYKDDDDK-tagged or FLAG-tagged proteins.