DDC and amyotrophic lateral sclerosis: Since treating diseases of the central nervous system is challenging due to the blood brain barrier (BBB), many AAV vectors, especially AAV1, AAV2, AAV5, AAV8, and AAV9, are found to be very useful in transducing neurons (200), and therefore, many AAV-mediated treatments are being tested to cure lysosomal storage disorders, Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis (ALS), epilepsy, spinal muscular atrophy type 1, metachromatic leukodystrophy, aromatic L-amino acid decarboxylase (AADC) deficiency, and Batten disease.