 The efficacy of clenbuterol treatment during ERT in patients with LOPD will be evaluated with muscle and pulmonary function testing as the primary endpoints. A secondary endpoint, the urinary Glc4 biomarker, will be monitored when the subjects are evaluated at baseline, week 18 and week 52. The impact of enhanced CI-MPR-mediated uptake of GAA will be analyzed by comparing the muscle function, pulmonary function, and biochemical correction of muscle in subjects with LOPD treated with ERT, both prior to and during simultaneous β2 agonist therapy. 