CD59, an LY6 like protein expressed in human lymphoid cells, regulates the action of the complement membrane attack complex on homologous cells. It is a potent inhibitor of the complement membrane attack complex action. It acts by binding to the C8 and/or C9 complements of the membrane attack complex, thereby preventing incorporation of the multiple copies of C9 required for complete formation of the osmolytic pore. This inhibitor appears to be species-specific. CD59 is also involved in signal transduction for T-cell activation complexed to a protein tyrosine kinase.