C9 and neoplasm: These complement inhibitors have also been shown to protect tumor cells against several therapeutic antibodies.15 CD46 has cofactor activity for inactivation of complement components C3b and C4b by serum factor I,16 whereas CD59 binds to C8 and C9, preventing C9 polymerization and insertion into membranes.17 There is substantial evidence in the literature that tumors18–22 - including MM23–25 - overexpress CD46 and CD59 and that these proteins limit the efficacy of mAb therapy.26