PTPRN and type 1 diabetes mellitus: With the autoantibody targets (pro)insulin, zinc-transporter 8 (ZnT8), glutamic acid decarboxylase 65kDa (GAD65) and insulinoma antigen-2 (IA-2) being intracellular and thus being inaccessible, the conundrum of autoantibodies in relation with T1D has not yet been resolved and a pathogenic role remains elusive.