Stem cell transplantation (SCT) has been performed in children with various metabolic diseases, including mucopolysaccharide disorders (MPS), leukodystrophies, and glycoprotein metabolic disorders.8 The prototypical metabolic disorder for SCT is Hurler syndrome, or MPS IH. Hurler syndrome results from the lack of a lysosomal enzyme needed to degrade glycosaminoglycans, which then build up and cause progressive intellectual decline, hepatosplenomegaly, cardiac valvular disease, airway abnormalities, and skeletal abnormalities. SCT performed early in the disease process leads to enzyme replacement and stops progression of disease.9 SCT has helped other MPS disorders including severe Maroteaux-Lamy syndrome and Sly syndrome, but unfortunately, and for reasons not well understood, SCT has not helped in several other MPS disorders, such as Hunter, Sanfilippo, and Morquio syndromes.8 X-linked adrenoleukodystrophy is a disease that affects boys with variable degrees of severity. When boys show early MRI changes and undergo SCT, outcome is very good, with preservation of neurological and neurocognitive function.10,11 Other leukodystrophies that respond to early SCT include Krabbe disease and metachromatic leukodystrophy.8,12 Glycoprotein metabolic disorders are exceedingly rare and thus there is limited experience with SCT in patients with these disorders. There have been successful outcomes in preservation of cognitive function in α-mannosidosis,13 fucosidosis,14 Batten disease,15 and Gaucher disease Type I.16 SCT has not been able to salvage children with Fabry disease, Tay Sachs disease, Pompe disease, and Sandhoff disease.8 Again, it is poorly understood why SCT is able to stop neurologic decline in some metabolic disorders and not others.