G6PC1 and hyperinsulinemic hypoglycemia, familial, 4: Two major subtypes are recognized: GSD type Ia, (GSD-Ia) caused by glucose-6- phosphatase (G6Pase) gene mutation leading to G6P deficiency and type Ib (GSSD-Ib) due to SLC37A4 gene mutation (2) due to defective glucose-6-phosphate transporter (G6PT) activity.