The monoclonal antibody MM10 recognizes membrane cofactor protein (MCP), also known as CD46. MCP is a type I membrane glycoprotein that consists of four isoforms i.e. two C-isoforms (51-58 kDa) and two BC isoforms (59-68 kDa) which arise by alternative splicing of a single CD46 gene. Membrane cofactor protein is expressed on every cell and tissue, with the exception of erythrocytes. Membrane cofactor protein serves to down-regulate the activation of complement on host tissue. MCP is a cofactor for factor 1-mediated cleavage of C3b and C4b and this way serves to protect the host cell against autologous attack. MCP is a receptor for several pathogens including measles virus, group A Streptococcus pyogenes and pathogenic Neisseria.