Another indication that Btn2a2 could play a role in bone homeostasis comes from measurements of Btn2a2 levels in sera of RA patients as well as from mice with a collagen-induced arthritis (CIA), in which the levels of Btn2a2 were significantly reduced in RA patients and CIA mice compared to normal healthy controls (Figures 4F, G), indicating that Btn2a2 is involved in diseases in which bone homeostasis is affected. The gene discussed is BTN2A2; the disease is rheumatoid arthritis.