Several endogenous proteins contain hydroxyphenylalanine and hydroxytyrosine residues. These residues are formed due to the hydroxylation of phenylalanine and tyrosine, a process in which the hydroxylation converts phenylalanine residues into tyrosine residues. This is very important in living organisms to help them control excess amounts of phenylalanine residues. Hydroxylation of tyrosine residues is also very vital in living organisms because hydroxylation at C-3 of tyrosine creates 3,4- dihydroxy phenylalanine (DOPA), which is a precursor to hormones and can be converted into dopamine.