PTPN22 and Autoimmunity: Different autoimmune diseases share susceptibility loci, but consistent associations with multiple autoimmune disorders have been restricted to three genes: the human leukocyte antigen (HLA) DRB1 gene, the PTPN22 gene encoding lymphoid tyrosine phosphatase LYP and the gene encoding cytotoxic T lymphocyte-associated 4 (CTLA-4) receptor [1].