FAP and arthritic joint disease: The fibroblast activation protein alpha+ (FAPα+) thymus cell antigen 1+(THY1+) FLS, located in the synovial sub‐lining, selectively promotes inflammation in arthritis with little effect on the bone and cartilage destruction and the FAPα+ THY1‐ FLS, located in the synovial lining layer, which selectively promote bone and cartilage impairment with little effect on inflammation.16