PTPRN and type 1 diabetes mellitus: Whilst the nature of the “64 k antigen” originally described by Baekkeskov et al. [9] was defined some years ago as two distinct proteins, the 65 kDa isoform of glutamate decarboxylase and IA-2 [10, 11], the identity of the 38 kDa component recognised by autoantibodies in type 1 diabetes has proved more elusive.