HLA-DRB1 and rheumatoid arthritis: The HLA-DRB1 gene constitutes the strongest genetic association linked to RA, and the allele associated with the disease is named as a shared epitope, which is a conserved sequence of five amino acids at positions 70–74 of the HLA-DRB1 gene [20], and this concept has been highly correlated with the ACPA-positive RA [33].