Amino acids are carboxylic acids with an amino-group in the α-position. With the exception of glycine, all amino acids are chiral; usually the L-form is used in living organisms. Twenty-two different amino acids are encoded in genes. Polycondensation of amino acids leads to peptides and proteins. The different side-chains of the various amino acids have different physicochemical properties and allow these amino acids to fulfil different functions inside a protein.