Lymphoid tyrosine phosphatase (LYP) encoded by the protein tyrosine phosphatase-22 (PTPN22) gene was originally associated with T1DM based on candidate gene approach [43] and subsequently shown to increase the risk for a number of autoimmune diseases including GD [44], rheumatoid arthritis (RA), juvenile idiopathic arthritis (JIA) and autoimmune Addison disease (AAD) [45]. This evidence concerns the gene PTPN22 and type 1 diabetes mellitus.