Subjects will be eligible regardless of whether they have begun enzyme replacement therapy prior to enrollment. All Subjects will receive enzyme replacement therapy as standard of care during the course of the Study, although they may not have begun ERT treatment at the time of enrollment. In addition to ERT, subjects may receive an immunomodulatory regimen as part of their standard of care; this may include rituximab, sirolimus, methotrexate, IVIg or other immunomodulatory agents such as pharmacological chaperone N-butyldeoxynojirimycin (NB-DNJ), alone or in combination, at the discretion of their caregiver(s).