The monoclonal antibody M177 recognizes CD46, also designated membrane cofactor protein (MCP). CD46 is a 45-70 kDa protein with genetic and tissue-specific heterogeneity.- It is expressed on every cell and tissue, with the exception of erythrocytes. CD46 serves to inhibit complement activation on host tissue. It performs this function by serving as a cofactor which binds to C3b and C4b. This binding is permitted by factor I, a serine protease of plasma, to degrade C3b and C4b and serves to protect the host cell against autologous attack. It also serves as a receptor for measles virus.Four isoforms of CD46 predominate and arise by alternative splicing of a single CD46 gene. CD46 cDNA encodes a signal sequence followed by four complement control protein domains (also called short consensus repeats (SCR)). The monoclonal antibody M177 reacts with the SCR2 domain.