Histidine (abbreviated as His or H; encoded by the codons CAU and CAC) is an ɑ-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated –⁺NH₃ form under biological conditions), a carboxylic acid group (which is in the deprotonated –COO^- form under biological conditions), and a side chain imidazole, classifying it as a positively charged (at physiological pH), aromatic amino acid. Initially thought essential only for infants, longer-term studies shown it is essential for adults also.