Double-staining of fibroblast activation protein (FAP) and smooth muscle actin (SMA). Synovial tissue samples from patients with destructive rheumatoid arthritis were stained to demonstrate the simultaneous expression of both antigens. FAP-positive fibroblasts in the intimal synovial lining were coloured brown (DAB [3,3'-diaminobenzidin]). Coexpression of SMA is visualised in red (fast red). Magnification, ×400.