Although HLA-DRB1 position 11 is shared between ACPA+ and ACPA− RA, the effects of individual amino acid residues are distinct; for example, a serine is protective for the development of ACPA+ RA, but increases the risk of developing ACPA− RA, whilst a glycine is protective for both subtypes. This evidence concerns the gene PRTN3 and rheumatoid arthritis.