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L-Proline is one of the twenty proteinogenic units which are used in living organisms as the building blocks of proteins. The other nineteen units are all primary amino acids, but due to the cyclic binding of the three-carbon side chain to the nitrogen of the backbone, proline lacks a primary amine group (−NH<sub>2</sub>). The nitrogen in proline is properly referred to as a secondary amine. Proline is sometimes called an imino acid, although the IUPAC definition of an imine requires a carbon-nitrogen double bond. In biological terminology, however, the category "amino acids" is generally taken to include proline.