The present invention relates to a one-vector expression system comprising a sequence encoding two polypeptides, such as tyrosine hydroxylase (TH) and GTP- cyclohydrolase 1 (GCH1 ). The two polypeptides can be should preferentially be expressed at a ratio between 3:1 and 15:1, such as between 3:1 and 7:1. The invention is useful in the treatment of catecholamine deficient disorders, such as dopamine deficient disorders including but not limited to Parkinsons Disease. Moreover, the present invention provides a method to deliver the vector construct in order to limit the increased production of the catecholamine to the cells in need thereof.
