PRTN3 and rheumatoid arthritis: Incubation of OA and RA cartilage sections with anti-human IgG-Texas Red (anti-hIgG-TR, Fig 4A and 4E) or with normal (ACPA-) human serum, followed by anti-hIgG-TR (Fig 4B and 4F) resulted in some background staining, likely due to the presence of trace amounts of human IgG in these cartilage specimens.