Histidine (His, H) is an amino acid. It is encoded by the codons CAU and CAC.
A string of 6-14 histidines is often called polyhistidine or His-tag.
The imidazole side chains of a His-tag can bind to certain transition metal ions (Ni, Co, Zn, Mg) whereas Nickle ion shows the highest affinity.
The classic His-tag has six consecutive histidines. Due to their hydrophilic and flexible nature, His-tags can increase the solubility of target molecules.