Abstract: The present invention provides methods for: (i) treating and/or preventing Parkinson's disease in a subject without causing cognitive impairment by using dopamine replacement gene therapy to maintain or restore constant physiological dopaminergic tone in both the dorsal and ventral striatum of the subject; (ii) normalising neuronal electrical activity in basal ganglia and/or subthalamic nucleus in a Parkinson's disease subject; and (iii) treating and/or preventing dyskinesias associated with oral L-dopa administration in a Parkinson's disease subject by administration of a vector system for dopamine replacement gene therapy to the subject.