For example, because approximately 50% of patients with RA display low/absent CD20+ B cells in diseased joint tissue (synovium)7, the target for the anti-CD20 rituximab monoclonal antibody, it has been postulated that the level of synovial B cells/B cell-related pathways would influence treatment response to rituximab. The gene discussed is MS4A1; the disease is rheumatoid arthritis.