Isoleucine is one of the 20 basic amino acids, and forms part of the structure of almost all proteins. It is coded for in DNA. Its chemical composition is identical to that of leucine, but the arrangement of its atoms is slightly different, resulting in different properties. Nutritionally, in humans, isoleucine is an essential amino acid. Isoleucine is a hydrophobic amino acid. Its three letter code is ILE and its one letter code is I.