Potent inhibitor of the complement membrane attack complex (MAC) action. Acts by binding to the C8 and/or C9 complements of the assembling MAC, thereby preventing incorporation of the multiple copies of C9 required for complete formation of the osmolytic pore. The mature form of this CD59 contains an additional serine residue before the conserved N-terminal leucine residue found in all other CD59 homologs sequenced to date.