Tyrosine hydroxylase (TH) catalyzes the conversion of tyrosine to L-DOPA, which is the rate limiting step in the biosynthesis of catecholamines such as dopamine, adrenalin and noradrenalin. Nerve cells with TH content are referred to as catecholaminergic cells and occurs mainly in the brain and adrenal glands. Defects in TH are the cause of Segawa syndrome, translating in progressive difficulty in walking. The activity of TH is also regulated by phosphorylation. Phospho-specific antibodies for the phosphorylation sites on TH can be used to great effect in studying this regulation and in identifying the cells in which TH phosphorylation occurs.