Rheumatoid arthritis (RA) is a complex di- sease in which environmental agents are thought to interact with genetic factors to trigger auto-immunity.
The contribution of genetic factors to RA susceptibility is well recognized. The heritability of anticitrullinated protein auto- antibody (ACPA)-positive and ACPA-negative RA implicates different genes . To date, the main known genetic factoris HLA, in particular the HLA-DRB1-shared epitope (SE) alleles, that predispose much more strongly to ACPA. However, the concordance for RA between monozygotic twins is only 15.6%. Thus, environment plays a crucial role in the development of the disease as well.