Using data from the Accelerating Medicines Partnership Rheumatoid Arthritis/Systemic Lupus Erythematosus (AMP RA/SLE) Consortium, Zhang et al. identified 4 synovial fibroblast subsets: a perivascular and interstitial THY1+/HLA-DRhi, perivascular sub-lining CD34+ fibroblasts, a DKK3+/CADM1+ sub-lining population and CD55+PRG4+ lining layer fibroblasts (36). Here, PRG4 is linked to rheumatoid arthritis.