PD is a neurodegenerative disorder mainly characterized by deficiency of the neurotransmitter dopamine and abundant tyrosine hydroxylase (TH) in the striatum of the brain, and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is known as a neurotoxin that impairs dopaminergic neurons. The gene discussed is TH; the disease is Parkinson disease.