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. This evidence concerns the gene FAP and osteoarthritis.