HLA-DRB1 and rheumatoid arthritis: To assess this, antibody responses to cit-TNC and arg-TNC peptides (plus the previously published cit-TNC5 peptide as a positive control) were measured in serum samples from HLA-DRB1*04:01 and cyclic citrullinated peptide+ (CCP+) RA (n = 17) and HLA-DRB1*04:01 matched HC (n = 24) subjects (Supplemental Table 2, Autoantibody Cohort 1, and Supplemental Table 5).