PRTN3 and rheumatoid arthritis: Another study examining ACPA-positive versus ACPA-negative RA in European-descent cohorts revealed that Ser-11 and Leu-11 in HLA-DRB1 and Asp9 in HLA-B can drive risk for ACPA-negative RA (51), potentially explaining our findings for *13:03 which also contains Ser-11.