alpha-Glucosidase is deficient (less than 30% of control) in Pompe's disease, but the extent of the deficiency does not always correlate with the severity of the clinical symptoms. The defects that lead to a deficiency of alpha-glucosidase include synthesis of catalytically inactive protein, absence of mRNA for the enzyme, decreased synthesis of the precursor, lack of phosphorylation of the precursor, impaired conversion of the precursor to the mature enzyme and synthesis of unstable precursor. A single type of defect can lead to different clinical phenotypes. The precursor of alpha-glucosidase is present in the brush border of the polarized epithelial cells of small intestine and kidney and is secreted into urine.