Native, human C9 complement component. Single-chain glycoprotein present in normal human serum at 60 μg/ml. On activation of complement via either the classical or alternative pathway, formation of the C5b fragment initiates assembly of C5b-9 complement membrane attack complex (MAC) on target surfaces. Full lytic activity of MAC occurs only after binding of three to six C9 molecules to each C5b-8 complex.