The SNP R620W, and the resultant risk allele, lymphoid tyrosine phosphatase (LYP) allele, is the most well-characterized risk variant of RA, as well as multiple autoimmune diseases, including type I diabetes, systemic lupus erythematosus, Hashimoto thyroiditis, Graves’ disease, Addison’s disease, myasthenia gravis, vitiligo, systemic sclerosis, juvenile idiopathic arthritis, and psoriatic arthritis [40]. Here, PTPN22 is linked to rheumatoid arthritis.