PRTN3 and rheumatoid arthritis: Several low abundance proteins in human serum, such as C-reactive protein [4], rheumatoid factor (RF) [5], anti-citrullinated peptide antibodies (ACPA) [6], and anti-keratin antibody (AKA) [7] have been investigated for detection of pre-RA symptoms [5], but none have been found to serve as a biomarker for RA initiation.