INS and retinal disorder: Most importantly, our healthy dogs were C-peptide positive, while the 7 T1D patients Oskarsson et al. studied were C-peptide negative, with 2 having “background retinopathy.” It is known from other studies of patients with T1D that glucagon secretion during insulin-induced hypoglycemia is absent in those who are C-peptide negative (27, 28), and that was also the case in the Oskarsson study; there was no rise in glucagon during hypoglycemia whether C-peptide was infused or not.