The goal of this natural history study is to collect clinical and biological information from patients with PMM2-CDG over 3 years. This study does not involve any investigational medications, but will provide information to researchers who are currently developing a medication to treat the disease. Participants in this study may be invited to take part in the upcoming drug study. This natural history study includes: physical exams, blood draws, ECG and Echos, developmental testing, ophthalmology exams, and liver assessments. If you have any of these procedures scheduled as routine appointments, you will not need a duplicate exam. If they are not part of your routine visits, the cost of exams will be provided free of charge. To learn more about the study, please contact our team via the information listed below.