A one-vector expression system comprising: - a first and a second expression cassette, wherein said first expression cassette comprises a nucleotide sequence comprising a first promoter sequence operably linked to a first nucleotide sequence, said first nucleotide sequence encoding a GTP-cyclohydrolase 1 (GCH1; EC 3.5.4.16) polypeptide or a biologically active fragment or variant thereof, and wherein said second expression cassette comprises a nucleotide sequence comprising a second promoter sequence operably linked to a second nucleotide sequence said second nucleotide sequence encoding a tyrosine hydroxylase (TH; EC 1.14.16.2) polypeptide or a biologically active fragment or variant thereof, and wherein the vector is an adeno associated vector and wherein said second nucleotide sequence is operably linked to a Woodchuck hepatitis virus post-transcriptional regulatory element (WPRE), and wherein both said first and said second promoter are Synapsin1 promoters, and wherein the GTP-cyclohydrolase 1 (GCH1; EC 3.5.4.16) polypeptide or the biologically active fragment or variant thereof is at least 70% identical to a polypeptide selected from the group consisting of SEQ ID NO. 1, SEQ ID NO. 2, SEQ ID NO. 3, SEQ ID NO. 4, SEQ ID NO. 5 and SEQ ID NO.6, wherein the biological activity is enzymatic activity of GTP-cyclohydrolase 1, and wherein the tyrosine hydroxylase (TH; EC 1.14.16.2) polypeptide or a biologically active fragment or variant thereof is at least 70% identical to a polypeptide selected from the group consisting of SEQ ID NO. 7, SEQ ID NO. 8, SEQ ID NO. 9, SEQ ID NO. 10, SEQ ID NO. 11, SEQ ID NO. 12, SEQ ID NO. 13 and SEQ ID NO. 14, wherein the biological activity is enzymatic activity of tyrosine hydroxylase. - a first and a second expression cassette, wherein said first expression cassette comprises a nucleotide sequence comprising a first promoter sequence operably linked to a first nucleotide sequence, said first nucleotide sequence encoding a GTP-cyclohydrolase 1 (GCH1; EC 3.5.4.16) polypeptide or a biologically active fragment or variant thereof, and wherein said second expression cassette comprises a nucleotide sequence comprising a second promoter sequence operably linked to a second nucleotide sequence said second nucleotide sequence encoding a tyrosine hydroxylase (TH; EC 1.14.16.2) polypeptide or a biologically active fragment or variant thereof, and wherein the vector is an adeno associated vector and wherein said second nucleotide sequence is operably linked to a Woodchuck hepatitis virus post-transcriptional regulatory element (WPRE), and wherein both said first and said second promoter are Synapsin1 promoters, and wherein the GTP-cyclohydrolase 1 (GCH1; EC 3.5.4.16) polypeptide or the biologically active fragment or variant thereof is at least 70% identical to a polypeptide selected from the group consisting of SEQ ID NO. 1, SEQ ID NO. 2, SEQ ID NO. 3, SEQ ID NO. 4, SEQ ID NO. 5 and SEQ ID NO.6, wherein the biological activity is enzymatic activity of GTP-cyclohydrolase 1, and wherein the tyrosine hydroxylase (TH; EC 1.14.16.2) polypeptide or a biologically active fragment or variant thereof is at least 70% identical to a polypeptide selected from the group consisting of SEQ ID NO. 7, SEQ ID NO. 8, SEQ ID NO. 9, SEQ ID NO. 10, SEQ ID NO. 11, SEQ ID NO. 12, SEQ ID NO. 13 and SEQ ID NO. 14, wherein the biological activity is enzymatic activity of tyrosine hydroxylase. a nucleotide sequence comprising a first promoter sequence operably linked to a first nucleotide sequence, said first nucleotide sequence encoding a GTP-cyclohydrolase 1 (GCH1; EC 3.5.4.16) polypeptide or a biologically active fragment or variant thereof, and a first promoter sequence operably linked to a first nucleotide sequence, said first nucleotide sequence encoding a GTP-cyclohydrolase 1 (GCH1; EC 3.5.4.16) polypeptide or a biologically active fragment or variant thereof, and said first nucleotide sequence encoding a GTP-cyclohydrolase 1 (GCH1; EC 3.5.4.16) polypeptide or a biologically active fragment or variant thereof, and a nucleotide sequence comprising a second promoter sequence operably linked to a second nucleotide sequence said second nucleotide sequence encoding a tyrosine hydroxylase (TH; EC 1.14.16.2) polypeptide or a biologically active fragment or variant thereof, and a second promoter sequence operably linked to a second nucleotide sequence said second nucleotide sequence encoding a tyrosine hydroxylase (TH; EC 1.14.16.2) polypeptide or a biologically active fragment or variant thereof, and said second nucleotide sequence encoding a tyrosine hydroxylase (TH; EC 1.14.16.2) polypeptide or a biologically active fragment or variant thereof, and