PTEN and obesity due to melanocortin 4 receptor deficiency: Although β-cell PTEN knockout mice have increased % β-cell/total pancreatic area (index of β-cell mass), as expected due to the chronic increase in growth-promoting pathways such as Akt and mTOR (25), their protection against obesity-induced diabetes in both of these models is due to improved β-cell function as evaluated by islet perifusion studies (25).