TH and Parkinson disease: Within the context of Parkinson’s disease, a neurodegenerative disease characterized by cognitive decline, Hommel et al. (2003) found that siRNA-mediated knockdown of Th, which is a gene responsible for the production of tyrosine hydroxylase and a key enzyme within the dopaminergic pathway, attenuated locomotor abnormalities in a mouse model of Parkinson’s disease.