ID1 and rheumatoid arthritis: Considering the known function of the CXCR6 receptor in relation to recruitment and homing of immune cells in RA [14], it is reasonable to expect that CXCR6 may also be involved in the recruitment and homing of Id1 expressing mesenchymal stem cells (MSCs) to RA synovium, likely for the purposes of tissue regeneration and/or vasculogenesis [17].