Acid maltase deficiency results in glycogen storage disease (GSD II) that may present in an infantile form (Pompe's Disease) or slowly progressive juvenile- or adult-onset form. The infantile form is rapidly fatal because of massive accumulation of glycogen in cardiac and skeletal muscle. Juvenile- or adult-onset forms are characterized by progressive skeletal muscle weakness and respiratory failure secondary to alveolar hypoventilation. Acid alpha glucosidase is a lysosomal enzyme that hydrolyzes linear a1-4 glucosidic linkages ranging from the large polymer glycogen to maltase. Martiniuk and colleagues cloned the 2856 bp cDNA in 1986 and have identified approximately 20 mutations in the enzyme that occurs in patients. The purpose of this study is to correlate acid maltase gene mutations in the enzyme that occurs in patients. The study will correlate acid maltase gene mutations with level of enzyme activity and course of disease. The laboratory will be used for oligonucleotide synthesis, DNA sequencing, DNA isolation, RNA isolation, PCR, Northern and Southern analysis, ultracentrifugation, and recombinant DNA techniques.