In seropositive RA, scRNA-seq studies revealed the expansion of fibroblast activation protein-α (FAPα)+ fibroblasts that comprised subsets of inflammatory FAPα+THY1+, and destructive FAPα+THY1− fibroblasts, respectively localized in the synovial sub-lining and lining layers (54, 55). Here, THY1 is linked to rheumatoid arthritis.