The same pattern of interaction was observed when RF (IgM and IgG) was used for the subdivision of RA cases: HLA-DRB1 SE alleles were found to interact significantly with smoking only for RF-positive RA (for IgM RF, OR = 23.5; 95% CI, 9.5 to 57.8; AP = 0.7, 95% CI, 0.6 to 0.9; and for IgG RF, OR = 19.4; 95% CI, 7.9 to 47.8; AP = 0.7, 95% CI, 0.5 to 0.9), but not for the RF-negative RA subgroup (data not shown). Here, HLA-DRB1 is linked to rheumatoid arthritis.