PTPN22 and type 1 diabetes mellitus: Recently, particular interest was generated by the potential pathophysiological role played in several autoimmune conditions including T1DM and APS3v [13] by the PTPN22 (protein tyrosine phosphatase N22 gene) C1858T mutation, which changes amino acid residue 620 from Arg (R) to Trp (W) (R620W) in the lymphoid tyrosine phosphatase Lyp protein.