The first subgroup includes the deficiency of brush border enzymes, and in particular congenital LD due to mutations within the gene encoding the protein with either lactase and phlorizin hydrolase activities [26]; congenital SID, due to mutations in the gene encoding the protein with both sucrase and isomaltase activity [8], and congenital maltase-glucoamylase deficiency (MGD), putatively due to the maltase-glucoamylase (MGAM) gene defect [8], even if some patients with the disease do not bear pathogenetic mutations in such gene. This evidence concerns the gene MGAM and glycogen storage disease II.