DDC (dopa decarboxylase) encodes an enzyme that catalyzes the decarboxylation of L-DOPA to dopamine and L-5-hydroxytryptophan to serotonin, representing a critical step in neurotransmitter biosynthesis 1. The enzyme is essential for the final step in dopamine synthesis and is involved in catecholamine and serotonin metabolic pathways 1. DDC operates through aromatic L-amino acid decarboxylase activity, converting aromatic amino acid precursors into their corresponding neurotransmitters. Common genetic variations in DDC have been shown to influence enzyme activity in the human brain, with specific haplotypes predicting DDC activity levels measured by PET imaging, particularly in the ventral striatum 2. Clinically, DDC deficiency causes aromatic L-amino-acid decarboxylase (AADC) deficiency, a severe neurological disorder characterized by autonomic, motor, and cognitive impairment 2. Gene therapy approaches using adeno-associated virus vectors to deliver the DDC gene have been developed as treatment, with eladocagene exuparvovec receiving approval for severe AADC deficiency 3. Additionally, DDC has emerged as a promising biomarker for early Parkinson's disease, with elevated levels in cerebrospinal fluid and urine correlating with clinical symptom severity and enabling accurate diagnosis of PD and prodromal PD 1.