You or your child are being asked to take part in this research study because you have Glycogen Storage Disease Type III (GSD III). The purpose of this research study is to better understand blood sugar variability, muscle strength and function, and impact of Glycogen Storage Disease Type III (GSD III) on overall health. You will not receive any treatment for your GSD III as part of this study, but the information you provide will be helpful for the study sponsor, Ultragenyx Pharmaceutical Inc., to develop a new therapy for GSD III.

If you agree to take part, your participation will last for about 26 weeks and will involve up to 2 study visits to the hospital along with up to 26 phone calls. There will be no changes to your treatment regimen for GSD III as part of this study.

As a participant in the research you will:

• come to the hospital for 1-2 in person visits to complete interviews and questionnaires,
• wear a blinded continuous glucose monitor (CGM) 3 months and an unblinded CGM for 3 months,
• complete fingerstick blood sugar checks, maintain a nutrition diary for 3 or more weeks throughout the study,
• maintain a hypoglycemia symptom diary, and
• upload CGM and HHG device data to a study-provided laptop weekly.

The main risks of this study are from hypoglycemia related to your diagnosis of GSD III and a loss of confidentiality. There are no direct benefits to you for participating in this study but the information collected as part of this study will help researchers better understand GSD III.