PRTN3 and rheumatoid arthritis: A study in Indigenous North Americans showed that the *14:02 allele confers substantial risk for RA due to a combination of serine residues at positions 11 and 13, which would allow HLA-DR to accommodate both citrulline and arginine at the P4 pocket, potentially enhancing anti-citrullinated antigen T cell responses and ACPA development (45).