This is a Phase 1/2, multicenter, open-label, single-ascending-dose study to evaluate the safety, tolerability, and efficacy of BEAM-301 in adult patients with GSDIa homozygous or compound heterozygous for the G6PC1 c.247C>T (p.R83C) variant and to determine the optimal biological dose.

Inclusion Criteria:

1. Males or females age ≥18 years of age at the time of consent.
2. Diagnosis of GSDIa and homozygous or compound heterozygous for the G6PC1 c.247C>T (p.R83C) variant (confirmed by genetic testing).
3. History of at least 1 episode of hypoglycemia <60 mg/dL within the 2 years prior to signing the ICF.

Exclusion Criteria:

1. Liver transplant recipient/on waiting list for liver transplant or known history of liver cirrhosis.
2. Presence of liver adenoma >5 cm in size based on liver MRI or liver ultrasound performed at screening or within 6 months prior to screening.
3. Presence of liver adenoma >3 cm and ≤5 cm with a documented annual growth rate of ≥0.5 cm per year.
4. Have aspartate transaminase or alanine transaminase >upper limit of normal (ULN).
5. Total bilirubin levels >ULN; if documented Gilbert's Syndrome, total bilirubin >2 × ULN.
6. Previous treatment with any cell-, gene-, or viral vector-derived therapy Hospitalization for management of hypoglycemia within 4 weeks prior to signing the ICF.
7. Have triglycerides >1000 mg/dL or a history of pancreatitis or a history of acute pancreatitis within 5 years that does not have a known and corrected etiology (eg, cholecystectomy for gallstone pancreatitis).