<unk>  (abbreviated as<unk>  or R) is an α-amino acid. The L-form is one of the 20 most common natural amino acids. Its codons are CGU, CGC, CGA, CGG, AGA, and AGG. In mammals,<unk>  is classified as a semiessential or conditionally essential amino acid, depending on the developmental stage and health status of the individual.  Infants are unable to effectively synthesize<unk> , making it nutritionally essential for infants.  Adults, however, are able to synthesize<unk>  in the urea cycle.<unk>  was first isolated from a lupin seedling extract in 1886 by the Swiss chemist Ernst Schulze.
Arginine
Phenylalanine
Citrulline
Asparagine
Ethionine
Tyrosine
Cysteamine 
Glutamine