Armaka et al. [12] found that THY1 and PRG4 were primarily expressed by distinct SF subsets in healthy joint tissue, but in RA, especially in disease-enriched S2d (Dkk3/Lrrc15+) and S4b (Birc5/Aqp1+) SFs, they were more likely to be coexpressed across both the sublining and lining compartments. Here, PRG4 is linked to rheumatoid arthritis.