Tyrosine (Tyr or Y) or 4-hydroxyphenylalanine is one of the 22 amino acids that are used by cells to synthesize proteins. It is a non-essential amino acid with a polar side group. Its codons are UAC and UAU. The word "tyrosine" is from the Greek tyros, meaning cheese, as it was first discovered in 1846 by German chemist Justus von Liebig in the protein casein from cheese. It is called tyrosyl when referred to as a functional group or side chain. Tyrosine is a hydrophobic amino acid.