Glycogen synthase deficiency, or glycogen storage disease type 0, is caused by mutations in the GYS2 gene that prevent the synthesis of glycogen in the liver. Symptoms of this condition usually appear in infancy or early childhood, and they include fasting hypoglycemia, ketosis, and low levels of lactate and alanine in blood. Hyperglycemia and hyperlacticacidemia often occur after eating. Some individuals with glycogen synthase deficiency may be relatively asymptomatic with occasional episodes of fasting hypoglycemia, while others may experience recurrent fatigue and vomiting, particularly during periods of fasting or illness. Severe episodes of hypoglycemia may present with pallor, drowsiness, confusion, seizures, and ultimately coma if left untreated. Muscle cramping, exercise intolerance, and minor growth delays may occur, but intelligence is generally not affected. Unlike many storage conditions, glycogen levels are decreased in the liver, so hepatomegaly is typically absent. Most symptoms can be relieved by dietary intervention. Glycogen synthase deficiency is inherited as an autosomal recessive condition and is believed to be significantly underdiagnosed.