MRAP and juvenile idiopathic arthritis: However, these specific markers can be demonstrated in only a portion of JIA subtypes, such as anti-cyclic citrullinated peptide antibodies and rheumatoid factor (RF) in polyarticular RF-positive JIA [4,5,6,7] and the human leukocyte antigen B27 in most patients with Enthesitis-Related Arthritis [8].