GYS2 and glycogen storage disorder due to hepatic glycogen synthase deficiency: Liver glycogen synthase deficiency (Fig. 1, E) (OMIM 240600), also known as GSD-0a, is caused by mutations in the GYS2 gene (OMIM 138571), which encodes the hepatic isoform of glycogen synthase and is located on chromosome 12p12.2.