MBTPS1 and rheumatoid arthritis: From a clinical point of view, S1P signalling has caught attention in inflammatory bone loss and resorption: Synovial fluid of patients with rheumatoid arthritis (RA) and extensive bone loss have significantly higher levels of S1P compared to patients with osteoarthritis,43 and S1P levels in serum correlated with inflammatory activity and bone erosion in patients with periodontitis.44