In PD, which is characterized by loss of dopaminergic neurons in the substantia nigra and a decrease in dopamine levels in the striatum, the transfer of genes that encode for several enzymes such as aromatic l-amino acid decarboxylase (AADC) that converts l-dopa to dopamine [16], glutamic acid decarboxylase (GAD) that modulates production of the neurotransmitter GABA (γ-aminobutyric acid) are all possibilities [17]. Here, DDC is linked to Parkinson disease.