CD34 and rheumatoid arthritis: In support of the latter, recent studies on the RA synovium revealed specific expression of Cthrc1 in two different FLS subpopulations, one of which, characterized by the lack of CD34 and the presence of THY1 and CDH11 (CD34− THY1+ CDH11+), is invasive, significantly expanded in RA vs. OA (38), and promotes osteoclastogenesis, which is a key contributer to RA pathogenesis by deregulating normal bone homeostasis in favor of bone resorption (33).