The invention provides nucleic acids encoding acid α-glucosidase (GAA). In certain embodiments, nucleic acids have greater than about 86% sequence identity to a sequence selected from the group consisting of any of the sequences set forth as SEQ ID NOs: 1-5. In certain embodiments, nucleic acids encoding acid α-glucosidase (GAA) contain less than 127 CpG dinucleotides. Expression cassettes, vectors, cells and cell lines and methods of using such nucleic acids encoding acid α-glucosidase (GAA) are also provided.