HLA-DRB1 and rheumatoid arthritis: According to this new classification, the susceptibility to RA, which depends on whether the RAA sequence occupies positions 72 to 74, was modulated by the amino acids at positions 70 and 71, which led to the definition of five groups of HLA-DRB1 alleles: S1, S2, S3P, S3D, and X alleles.