Histidine is one of the 23 amino acids used to build proteins in humans, known chemically as proteinogenic amino acids. It uses the codons cytosine-adenine-uracil (CAU) and cytosine-adenine-cytosine (CAC). The German physician Albrecht Kossel first isolated histidine in 1896. It was first believed to be an essential amino acid only in infants, although subsequent research has now established histidine as an essential amino acid for all humans.