The FAPα-expressing protein in the combination of podoplanin (PDPN) and CD90 (also known as thymus cell antigen 1, THY1) enabled synovial cells in the sub-lining layer (FAPα+PDPN+THY1+) to be distinguished from the synovial cells in the lining layer (FAPα+PDPN+THY1−) in human RA tissue [116]. Here, PDPN is linked to rheumatoid arthritis.