PTPN22 and type 1 diabetes mellitus: The C1858T single nucleotide polymorphism in the human protein tyrosine phosphatase non-receptor type 22 (PTPN22) gene encodes an R620W missense mutation in PTPN22 and is associated with an increased susceptibility to multiple autoimmune diseases including rheumatoid arthritis (RA)1–4, systemic lupus erythematosus (SLE)5,6 and type 1 diabetes (T1D)7–9.