Using the Sgsh−/− mouse model (hereafter called MPS-IIIA), which bears a spontaneous mutation in the sulfamidase gene, we have investigated changes in the development and function of the meso-striatal dopaminergic system in MPS-IIIA and their sensitivity to dopaminergic drugs. The gene discussed is SGSH; the disease is mucopolysaccharidosis type 3A.