Aromatic L-amino acid decarboxylase deficiency (AADCD) is a rare neurotransmitter disorder. It is caused by the presence of pathogenic variants of the DDC (Dopa Decarboxylase) gene that cause the disease. Hereditary AADC enzyme deficiency causes a marked loss of dopamine and serotonin production in the brain from birth. AADC is the final enzyme in the biosynthesis of dopamine and serotonin. AADC deficiency leads to metabolic disorders of dopamine, norepinephrine, epinephrine and serotonin. Genetic variants of the Dopa Decarboxylase (DDC) gene encoding AADC cause decreased serotonin synthesis as well as dopamine and catecholamine, while causing increased 3-methoxy-L-tyrosine (3-OMD) concentration. Positive treatment outcomes are associated with early, pre-symptomatic diagnosis and initiation of treatment, and increased 3-OMD concentration in dried blood spots of patients with AADC deficiency makes 3-OMD a candidate biomarker for newborn screening (NBS).