1981; Ferrannini et al. 1985). These processes are tightly regulated by hormones and circulating substrate levels (Roden and Bernroider 2003). Insulin stimulates glycolysis and suppresses gluconeogenesis but promotes glucose storage in the liver as glycogen under hyperglycemia. In contrast, glucagon suppresses glycolysis and stimulates gluconeogenesis. In various diseases the liver glucose homeostasis is of pivotal importance, especially in type 2 diabetes mellitus (T2DM) and nonalcoholic fatty liver disease (NAFLD) (Roden and Bernroider 2003). Here, GCG is linked to type 2 diabetes mellitus.