DOPA decarboxylase (DDC), also known as aromatic-L-amino acid decarboxylase, is a 54 kDa lyase enzyme. It is a vitamin B6-dependent homodimeric enzyme that catalyzes the decarboxylation of both L-3, 4 -dihydroxyphenylalanine (L-DOPA) and L-5-hydroxytryptophan (5-HTP) to dopamine and serotonin, respectively. In normal dopamine and serotonin neurotransmitter synthesis, DDC is not the rate-limiting step in either reaction. However, DDC becomes the rate-limiting step of dopamine synthesis in patients treated with L-DOPA (such as in Parkinson’s Disease), and the rate-limiting step of serotonin synthesis in people treated with 5-HTP (such as in mild depression or dysthymia). DDC is inhibited by Carbidopa outside of the blood brain barrier to inhibit the premature conversion of L-DOPA to dopamine in the treatment of Parkinson’s. In humans, DDC is also the rate-limiting enzyme in the formation of trace amine neurotransmitters. Deficiency of DDC is associated with various symptoms as severe developmental delay, oculogyric crises and autonomic dysfunction.