Several MHC class II (MHC-II) alleles, e.g. HLA-DRB1*0401, are associated with severe rheumatoid arthritis (RA). The relevant polymorphisms map to the antigen binding groove, but unique arthritogenic peptides have not been identified. In antigen presenting cells, peptide loading by MHC class II is facilitated by HLA-DM, an MHC-II chaperone and peptide exchange catalyst. Generally, expression of DM (DMA/DMB) genes and of MHC-II is co-regulated, but distinct promoter regions allow for some differential regulation. In some studies, structural polymorphisms in DM have also been associated with RA in a small subset of patients. Study aims were to identify and functionally characterize polymorphisms in the DMA and DMB promoters, and to compare DM levels in RA patients and controls.