THY1 and rheumatoid arthritis: The THY1 membrane glycoprotein (CD90) was used as a general marker for distinguishing two pathogenic fibroblast subsets related to the RA: THY1+ fibroblasts (located in the synovial sub-lining) that increase the severity and persistence of inflammation, and THY1− fibroblasts (located in the synovial lining) that are responsible for cartilage and bone destruction [6,25,26,28,29].