The FcγRIIIA-stimulating activity of the ICs generated by mixing U1RNP complex with serum IgG from anti-U1RNP autoantibody–positive MCTD or SLE patients was significantly decreased by RNase treatment, but the effect of RNase was inhibited by RNase inhibitor. The gene discussed is FCGR3A; the disease is systemic lupus erythematosus.