Azetidine-2-carboxylic acid (abbreviated Aze) is a plant non-protein amino acid homologue of proline with the molecular formula C4H7NO2. Aze is a heterocyclic, 4 membered ring with nitrogen as its heteroatom (an azetidine), and a carboxylic acid group substituted on one of the ring carbon atoms. The main difference between Aze and proline is the ring of Aze has four members and the ring of proline has five. Aze has the ability to act as an analog of proline and can be incorporated into proteins in place of proline.