FAP is exclusively expressed in fetal cells but not expressed in healthy adult tissue, except bone marrow derived mesenchymal stem cells and wounded tissues.[6, 7, 8] In the RA synovium, FAP is highly expressed by fibroblasts in the lining and sub-lining layers.[4,9] The expression is highly specific to RA fibroblasts since FAP is expressed in low levels in osteoarthritis and is not expressed in normal synovial fibroblasts. The gene discussed is FAP; the disease is rheumatoid arthritis.