This variant was found in 2 Japanese individuals (the proband (55yo) and his son (22yo)) in a heterozygous manner with glycogen storage disease X and exercise intolerance. The variant was also present in a 19yo son and 18yo daughter who are currently asymptomatic. It was not seen in 20 ethnically matched controls. The authors conclude pathogenecity in this family due to evolutionary conservation, no other mutations present, no other biochemical defect detected, not in controls and a predicted strain on the secondary structure sufficient to alter the active site. Most variants in the gene are recessive, but manifesting heterozygosity has been noted.