HLA-DRB1 and rheumatoid arthritis: The SNP, rs6933349, which is in the region of MHC class I (Additional file 1: Figure S1), had no evidence of LD with HLA-DRB1 SE alleles (r2 ≤ 0.05) (Additional file 1: Table S1), suggesting that it represents a novel gene-environment interaction in the risk of developing ACPA-positive RA.