CXCL13 and synovial sarcoma: Five proteins (C-X-C motif chemokine 13 (CXCL13), tumor necrosis factor receptor 2 (TNF-R2), CD48, B-cell activating factor (BAFF), and programmed cell death protein 1 ligand 2 (PD-L2)) related to the disease activity of SS, including the B-cell activating factor (BAFF) [23], which was recently identified by the European League Against Rheumatism (EULAR) as a biomarker that is correlated with the European League Against Rheumatism Sjögren Syndrome Disease Activity Index (ESSDAI) score, have been identified in serum.