As previously described, lining fibroblasts (F-0 and F-1) were depleted in rheumatoid arthritis relative to osteoarthritis and subdivided into PRG4+CLIC5+ (F-0), PRG4+ (F-1) and RSPO3+ (F-8) populations, the last exhibiting an intermediate lining–sublining phenotype. The gene discussed is PRG4; the disease is rheumatoid arthritis.