Stratification for serum anti-CCP antibody showed risk effect of HLA-DRB1*04 and protective effect of allele *13 in RA patients [see Table S16 in Additional data file 1] even after correction for multiple testing (Pcorr = 0.025 and Pcorr = 0.036, respectively). This evidence concerns the gene HLA-DRB1 and rheumatoid arthritis.