CD28 and rheumatoid arthritis: We have previously tested markers of anti‐CCP–positive RA for their association with anti‐CCP–negative RA 8 and reported that several anti‐CCP–positive RA susceptibility loci (e.g., AFF3, CCR6, CCL21, IL2RA, and CD28) were not shared with anti‐CCP–negative RA, while markers at TNFAIP3, C5orf30, STAT4, ANKRD55, BLK, and PTPN22 were associated with both anti‐CCP–positive and anti‐CCP–negative RA.