DDC and Parkinson disease: Like other PLP enzymes [1–7], DDC is of clinical interest since it is involved either in Parkinson's disease, a degenerative disorder of the central nervous system resulting from the death of dopamine-generating cells in the substantia nigra, or in AADC deficiency, a rare inherited neurometabolic disease due to mutations on the AADC gene leading to deficit of catecholamines and serotonin in the central nervous system and periphery [8].