Serine (abbreviated as Ser or S) encoded by the codons UCU, UCC, UCA, UCG, AGU and AGC is an ɑ-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated − form under biological conditions), a carboxyl group (which is in the deprotonated – form under biological conditions), and a side chain hydroxyl group, classifying it as a polar amino acid. It is non-essential in humans, meaning the body can synthesize it.