PDPN and rheumatoid arthritis: Among those markers, cadherin-11 (CDH-11), fibroblast activation protein α (FAPα) and podoplanin (PDPN/GP38) are considered as general markers that associated with the pathogenic FLS phenotype in RA [14, 35]; however, FLSs in the physiological state may also express these markers but at relatively low levels.