PTPN22 and systemic lupus erythematosus: Multiple studies have identified genes involved in immune-related functions to be under selection [8–10], including the HLA [11–14] (associated with all autoimmune diseases), BTLA [10] (associated with rheumatoid arthritis), ITPR3 [10] (SLE, type 1 diabetes, Grave's disease), PTPN22 [10] (rheumatoid arthritis, Crohn's disease, type 1 diabetes, vitiligo), ITGAX [10] (SLE), and BLK [10] (SLE, rheumatoid arthritis, Kawasaki disease).