hereditary deficiency of the liver enzyme glycogen phosphorylase, which governs the metabolic breakdown of glycogen to the simple sugar glucose, which can then be used to meet the body's energy needs. The enzyme's absence causes glycogen to accumulate, greatly enlarging the liver and producing moderate hypoglycemia (low blood sugar), since the release of glucose from storage in the liver is impaired. Other symptoms of the disease are a mild increase in the acidity of the blood and growth retardation. No other organs seem to be involved. Unlike some of the related glycogen storage diseases, Hers' disease does not cause mental retardation or reduce the life span.