Subjects enrolled in this natural history study will be thoroughly examined for signs and symptoms of PMM2-CDG. Medical history, physical examination, laboratory testing and imaging studies will be performed during a single consultation. Follow-up will occur every 6 months at a minimum, depending on the standard of care at the investigator's institution as well as the clinical status of the individual patient. All medical procedures are routine. No new therapy is offered as part of this study, and no change in the patients' routine therapy is dictated by this protocol. No randomization will be performed.