Provided herein are methods and compositions for treating a subject sufferingfrom a deficiency in α-L-Iduronidase in the CNS. The methods include systemicadministration of a bifunctional fusion antibody comprising an antibody toa human insulin receptor and an α-L-Iduronidase. A therapeutically effectivesystemic dose is based on the specific CNS uptake characteristics of human insulinreceptor antibody-α-L-Iduronidase fusion antibodies as describedherein.