PTPN22 and rheumatoid arthritis: Most importantly, PTPN22, which encodes lymphoid-specific tyrosine phosphatase (Lyp), attracts tremendous attentions due to the linkage of its R620W single nucleotide polymorphism (SNP) to many autoimmune diseases, including Type 1 Diabetes [5], [6], rheumatoid arthritis [7], [8], and systemic lupus erythematosus [9], [10].