Pathological hallmark of PD is loss of dopaminergic neuron in the pars compacta of the substantia nigra (SNc) [13, 14]; PD patients display reduction of dopamine levels in striatum and degeneration of the dopamine producing neurons, because of the catalytic rate limiting step of tyrosine hydroxylase in catalyzing the conversion of the amino acid tyrosine to L-3,4-dihydroxyphenylalanine (L-DOPA). The gene discussed is TH; the disease is Parkinson disease.