PTPN22 and rheumatoid arthritis: Examples of genes implicated by both common and rare genetic variation to date are CTLA4, with known roles in rheumatoid arthritis (RA), type 1 diabetes (T1D), and Hashimoto’s thyroiditis (HT) [1–3], PTPN22 in systemic lupus erythematosus (SLE), T1D, RA and Grave’s disease (GD) [4, 5], and IL23R in ulcerative colitis (UC), Crohn’s disease (CRO), SLE, RA and psoriasis (PSO) [6–8].