Pompe disease (MIM # 232300) is an autosomal recessive lysosomal glycogen storage disorder first described in 1932 (1) and it is caused by biallelic mutations in the GAA gene (MIM 606800) that encodes the lysosomal enzyme acid alpha-1,4-glucosidase (GAA; EC 3.2.1.20), also known as acid maltase. Here, GAA is linked to disorder of glycogen metabolism.