Thus, while in some cases classification of arthritis is rather straight-forward, e.g., through the presence of anti-citrullinated protein antibodies (ACPA) (3) in RA or the presence of plaque psoriasis in PsA (4), many forms of arthritis remain undefined and are therefore termed as undifferentiated arthritis (UA) (5). This evidence concerns the gene PRTN3 and Arthritis.