3-Methylornithine is an amino acid with the formula H<sub>2</sub>N(CH<sub>2</sub>)<sub>2</sub>CH(CH<sub>3</sub>)CH(NH<sub>2</sub>)CO<sub>2</sub>H. This amino acid contains two stereogenic centers, but only one stereoisomer (namely (3R)-3-methyl-<small>D</small>-ornithine) occurs in nature. It is produced from lysine by the action of the enzyme methylornithine synthase. The combination of lysine and 3-methylornithine, also mediated enzymatically, produces pyrrolysine, which, for some organisms, is a 22nd genetically coded amino acid.