After the pharmacological induction of PD-like symptoms, the monkeys were treated with the mix of three viruses expressing enzymes necessary for the synthesis of dopamine (tyrosine hydroxylase (TH), aromatic L-amino acid decarboxylase (AADC) and guanosine triphosphate cyclohydrolase I (GCH)) via an injection into the left putamen; in the right side, PBS was injected as a control. Here, TH is linked to Parkinson disease.