For instance, we identified 3 children who phenotypically resembled seropositive RA, (two positive tests for RF-IgM, and positive for ACPA, carriage of the HLA DRB1 shared epitope) but had to be classified as “undifferentiated JIA” due to the specific requirements of the ILAR criteria. The gene discussed is PRTN3; the disease is juvenile idiopathic arthritis.