Glycogen storage diseases (GSD) type 1b is the first example of a genetic disorder involving the transport system of an intracellular membrane. It was revealed that the primary defect in GSD type 1b was a deficiency in the microsomal glucose-6-phosphate (G6P) translocase, based on the findings that the glucose-6-phosphatase activity was highly latent in the fresh liver homogenates. Further evidence of this defect in GSD type 1b has been provided by a membrane filter method which measures the uptake of 14C-G6P by microsomes. The clinical symptoms and enzymatic studies in our patients suggest that there is genetic heterogeneity in GSD 1b and the clinical severity depends on the level of residual activities of G6P translocase.