HLA-DRB1 and rheumatoid arthritis: We observed that ever-smokers carrying the HLA-DRB1*04:05 allele had a 12.9-fold increased risk of ACPA-positive RA, and that signs of interaction were found between smoking and HLA-DRB1*04:05, although the interaction was not statistically significant (AP = 0.4; 95% CI, -0.1 to -0.9).