Genetic variants of transporters contributing to intestinal sugar transport are associated with human diseases, such as glucose-galactose malabsorption and Fanconi-Bickel syndrome caused by mutations in SGLT1 (SLC5A1) and GLUT2 (SLC2A2), respectively (Martin et al., 1996; Santer et al., 1997). Here, SLC2A2 is linked to glycogen storage disease due to GLUT2 deficiency.