The HA tag is a nine-residue epitope taken from influenza virus haemagglutinin and one of the small handful of tags that dominate routine protein work. The supplied molecular weight of about 1102 is consistent with the standard YPYDVPDYA sequence. Its usefulness comes from being short, well tolerated at either terminus of a recombinant protein, and matched to long-established monoclonal antibodies, of which 12CA5 is the most widely cited. Because the epitope is viral it is absent from mammalian proteomes, so anti-HA antibodies give low background on cell lysates, and the tag is small enough that it rarely perturbs folding or activity in the way a fluorescent protein fusion can. The free peptide has a specific role distinct from the tagged protein itself. It is used as a competing ligand to elute intact tagged protein from an anti-HA affinity resin under gentle conditions, avoiding the low pH or denaturant that would otherwise be needed and preserving activity and complex integrity, which matters for co-immunoprecipitation. It also serves as a blocking peptide to confirm that an antibody signal is epitope-specific: pre-incubating the antibody with excess free peptide should abolish the band or stain, and a signal that survives that competition is not coming from the tag. Both uses call for the free peptide rather than a tagged protein standard. | 