The present invention provides a construct comprising (i) a nucleotide sequence which encodes tyrosine hydroxylase (TH), (ii) a nucleotide sequence which encodes GTP-cyclohydrolase I (CH1) and (iii) a nucleotide sequence which encodes Aromatic Amino Acid Dopa Decarboxylase (AADC) wherein the nucleotide sequence encoding AADC is linked to the nucleotide sequence encoding TH such that they encode a TH-AADC or AADC-TH fusion protein. The invention also provides a viral vector comprising such a nucleotide sequence and its use in the treatment and/or prevention of Parkinson's disease.
