SB203580 administration before disease onset results in increased incidence of arthritis. Immunohistochemistry of phosphorylated p38 (p–p38) in ankylosing enthesitis. Immunoreactivity for p–p38 is detected in entheseal fibroblast-like cell (arrowheads), prehypertrophic chondrocytelike cells (arrows) and bone. Negative control staining using normal rabbit IgG. Magnification 10× (1) and 20× (2–3). p 726.