The HAT tag, or Histidine Affinity Tag, is a peptide epitope (KDHLIHNVHKEFHAHAHNK) used in protein expression and purification. Similar to the His tag, it is rich in histidines and has high affinity for immobilized metal ions such as cobalt, nickel, and zinc. However, the sequence is naturally-occurring, featuring non-adjacent His residues that result in a lower overall charge than tags with consecutive histidines. The HAT tag is meant to more closely resemble the solubility of wild-type proteins. HAT tag antibodies are available for the detection of tagged fusion proteins.