As expected, the number of COX-2+ osteocytes decreased significantly (Fig. 5b, c), and the number of osteocalcin+ osteoblasts and TRAP+ osteoclasts did not change significantly compared with that in controls (Supplementary Fig. 3a–c), but the number of osterix+ osteoprogenitors in COX-2−/− RA mice decreased relative to that in controls (Supplementary Fig. 3a, d). Here, ACP5 is linked to rheumatoid arthritis.