PRTN3 and rheumatoid arthritis: We observed that children who were positive for both RF-IgM and ACPA phenotypically resemble adults with RA, and also encompass the typical child with polyarticular “RF positive JIA.” We propose that in children who are RF-IgM negative, a higher threshold for ACPA-positivity might be prudent in order to identify those children who have a phenotype compatible with RA.