A hormone that is produced in the body under various circumstances, resulting in noticeable effects on behaviour. It is a peptide made up of nine amino acids in the order: cysteine – tyrosine – isoleucine – glutamine – asparagine – cysteine – proline – leucine – glycine – amide, with a disulphide bridge joining the cysteine moieties. Oxytocin was discovered in 1906 and was the first hormone to have its amino acid sequence determined and to be synthesized, with both being achieved in the early 1950s.