Lysosomal storage disorders are rare, inherited diseases caused by a deficiency of a specific, catabolic, lysosomal enzyme. In the case of mucopolysaccharidosis type IIIA, a lack of active sulfamidase enzyme results in heparan sulfate accumulation, severe and progressive neurological deficits, and usually premature death. Embryonic stem cells can be genetically modified to overexpress lysosomal enzymes, providing a renewable reservoir of cells that can be readily expanded in culture. Screening clonal lines of embryonic stem cells for desirable properties such as high levels and maintenance of enzyme activity throughout terminal differentiation to neural phenotypes theoretically provides a reproducible population of cells that can be fully characterized in vitro before implantation within the central nervous system in animal models of lysosomal storage disorders.