DDC and Parkinson disease: After the promising results of hAADC gene therapy in PD patients, a similar approach has been trialed in patients with aromatic L-amino acid decarboxylase deficiency (AADCD; OMIM #608643), a rare autosomal recessive neurotransmitter disorder that leads to a severe combined deficiency of serotonin, dopamine, norepinephrine and epinephrine.