Glycogen storage disease type Ib (GSD Ib) (Orphacode 79,259) is a rare autosomal recessive disease, with an estimated prevalence of 1: 500,000, due to mutations in SLC37A4 encoding the glucose-6-phosphate transporter (G6PT) of the endoplasmic reticulum. The gene discussed is SLC37A4; the disease is glycogen storage disease Ib.