Mucopolysaccharidosis type I (MPS I) is an autosomal recessive disease which leads to systemic disease, including progressive neurodegeneration, mental retardation and death before the age of 10 years. MPS I results from deficiency of α-L-iduronidase (IDUA) and subsequent accumulation of glycosaminoglycans (GAG). IDUA enzyme acitivity is an essential assessment for research and diagnostic testing of MPS I disease. Due to different parameters (reaction time, temperature and substrate concentration) used by different labs, the enzyme levels of a certain sample varied. To solve the inconsistency of IDUA enzyme assays in this field, a standardized protocol of IDUA enzyme assay was established through adjustment by Michaelis-Menten equation (Chaper 1). In clinical practice, MPS I disease is treated by enzyme replacement therapy (ERT) and bone marrow transplantation (BMT). Clinical ERT with intravenous IDUA reverses some aspects of MPS I disease and ameliorates others. However, neurologic benefits are thought to be negligible because the blood-brain barrier (BBB) blocks enzyme from reaching the central nervous system (CNS). To address this question, high-dose IDUA (11.6 mg/kg, once per week, 4 weeks) was administered to adult MPS I mice. IDUA enzyme activity in cortex of injected mice increased to 97% of that in wild type mice (p<0.01). GAG levels in cortex were reduced by 63% of that from untreated MPS I mice (p<0.05). Water T-maze tests showed that the learning abnormality in MPS I mice was surprisingly reduced (p<0.0001). These results demonstrated the efficacy of high dose ERT in treating neurological diseases in MPS I mice (Chapter 2). Previous study in our lab showed that a single administration of lentiviral vector in neonatal MPS I mice can achieve significant metabolic correction and neurological improvements. To further improve the efficacy of lentiviral gene therapy, a total of 9 constructs were designed by codon optimization, and different combination of promoters and enhancers. The transgene expression of these 10 constructs was compared after transfection into HEK 293FT cells, and 5 constructs with the highest IDUA expression were identified (Chapter 4). These results pave the way for developing a directly applicable clinical trial of human lentiviral gene therapy for MPS I disease, and also provide evidence for vector design for treating other lysosomal diseases.