It is known that amino acid position 13 is involved in shaping the peptide-binding pocket 4 of HLA-DRB1.16 We find that the association in the combined oligoarthritis and RF-negative polyarthritis dataset mirrors the findings seen in seronegative RA and similarly, in RF-positive polyarthritis, the findings correspond to the association in seropositive RA. Here, HLA-DRB1 is linked to polyarticular arthritis.