Susceptibility to rheumatoid arthritis (RA) maps to residues QKRAA/QRRAA in the third hypervariable region of the HLA DR beta 1 chain. Peptides from the same area of MHC class II molecules are able to modulate the T-cell repertoire by deleting self-reactive T-cells. The Epstein Barr virus glycoprotein gp110 and the dna J heat-shock protein from E. coli mimic the third hypervariable region of HLA-Dw4DR beta 1. Thus, the same area of HLA DR beta 1 carries susceptibility to RA, modulates the T-cell repertoire and is mimicked by human pathogens. RA may originate from a particular shape imposed on the T-cell repertoire by the QKRAA/QRRAA sequence in the third hypervariable region of HLA DR beta 1.