Glycogen storage disease (GSD) type Ib is an autosomal recessive disorder caused by a deficiency in microsomal glucose 6-phosphate (G6P) translocase. A gene mutated in GSD type Ib patients has recently been isolated. We have determined the entire sequence of the human G6P translocase gene by PCR-directed sequencing. The gene spans approximately 5 kb of genomic DNA and contains eight exons. Analysis of DNA from a Japanese patient with GSD type Ib revealed new compound heterozygous mutations; a T to C transition at cDNA position 521 resulting in W118R, and an A to C transversion at the -2 splicing acceptor site of intron 1. Reverse transcription (RT)-PCR from leukocyte RNA of the patient revealed the abnormally spliced transcript. These results further support the suggestion that the gene is causative for GSD Ib and should be useful in the molecular diagnosis of such patients.