 This study will measure the incidence of Pompe disease based on manifest laboratory abnormality, namely low GAA enzyme activity. Analysis of GAA enzyme activity will be determined through a blood sample of 4 mL. The study seeks to measure the epidemiology of Pompe disease by symptomatically screening all patients who present with symptoms of hitherto undiagnosed proximal weakness with or without elevation of the muscle enzyme, creatinine kinase CK, or elevation of CK alone, at thirteen academic tertiary neuromuscular practices throughout the United States and Canada. Total recruitment is expected to be 1,500 participants. It is anticipated that the number of incident Pompe cases in this cohort would be between 3-5, i.e. 45-75 newly diagnosed cases of Pompe disease. 