Omega hydroxy acids (also known as ω-hydroxy acids) are a class of naturally occurring straight-chain aliphatic organic acids n carbon atoms long with a carboxyl group at position 1 (the starting point for the family of carboxylic acids), and a hydroxyl at terminal position n where n > 3. The C16 and C18 omega hydroxy acids 16-hydroxy palmitic acid and 18-hydroxy stearic acid are key monomers of cutin in the plant cuticle. The polymer cutin is formed by inter-esterification of omega hydroxy acids and derivatives of them that are substituted in mid-chain, such as 10,16-dihydroxy palmitic acid. Only the epidermal cells of plants synthesize cutin.