Alpha-mannosidosis (AMD) is an inherited lysosomal storage disorder caused by mutations in the LAMAN gene, which encodes lysosomal alpha-mannosidase and is characterized by neurodevelopmental delay, mild immune deficiency, facial and skeletal abnormalities, hearing impairment, intellectual disability, muscle weakness and ataxia. The progression of neuromuscular and skeletal deterioration is insidious, occurring over several decades, rendering most patients wheel-chair dependent. No consistently successful treatment is available. To better characterize the biochemical phenotype and natural history of this disorder, we will study 15 patients with AMD, ranging in age from five to 60 years, recruited from Departments of Biochemical Genetics and Medical Genetics at university medical centers mainly in the US and Canada or referred by the Intl Society for Mannosidosis & Related Diseases. Participants in the study will visit the NIH Clinical Center for 2-3 days during which they will undergo clinical and biochemical evaluations to establish reliable clinical benchmarks and to identify cerebrospinal fluid biomarkers that could serve as candidate surrogate markers of treatment effect in future clinical trials. The protocol will take advantage of the NICHD Biomedical Mass Spectrometry Facility to generate CSF proteomic profiles. Patients will also undergo MR spectroscopy (under sedation/anesthesia, if appropriate) in order to establish the phenotypic baseline and for possible utility as a guide for natural history and/or treatment outcomes in future studies. If the pre-clinical components of this proposal prove promising, the prospect of a recombinant adeno-associated viral gene therapy trial involving a brain-directed (intrathecal) approach for AMD would be possible within 3 years.