S100A12 and rheumatoid arthritis: The present study is the first to address these issues systematically and quantitatively in a large set of actively inflamed synovium samples by (1) conducting a detailed analysis of PsA synovitis in comparison with AS–USpA on the one hand and RA on the other, using a large panel of histopathological features; (2) comparing oligoarticular versus polyarticular PsA with respect to synovial features that discriminate between RA and SpA; and (3) analyzing the expression of S100A12, which was reported to be an interesting marker in PsA [10].