HLA-DRB1 and rheumatoid arthritis: Among the genetic risk factors, the shared epitope (SE), a sequence of a five-amino acid motif encoded by HLA-DRB1 alleles, is the most significant one.48 The SE is associated with an increased RA susceptibility, an earlier disease onset, more severe cartilage erosions, and the presence of ACPAs.49 Distinct genetic patterns between the seropositive and seronegative subsets of the disease have been defined according to ACPA status.50