Provided are retroviral vector genomes and vector systems comprising the genomes. In particular, a retroviral vector genome comprising two or more NOIs, operably linked by one or more Internal Ribosome Entry Site(s); a lentiviral vector genome comprising two or more NOIs suitable for treating a neurodegenerative disorder; and a lentiviral vector genome which encodes tyrosine hydroxylase, GTP-cyclohydrolase I and, optionally, Aromatic Amino Acid Dopa Decarboxylase are provided.