The monoclonal antibody MM.1 recognizes CD46, also known as membrane cofactor protein. An immunohistological survey of rat tissues revealed that CD46 is expressed only in the testis and epididymis of adult rats more specifically, only by spermatozoa and late spermatozoal precursors.In contrast, in humans, CD46 is expressed on every cell and tissue, with the exception of erythrocytes. CD46 serves to down-regulate the activation of complement on host tissue. It functions as a cofactor by binding to C3b and C4b. 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. In human, it also serves as a receptor for measles virus.Two isoforms of CD46 predominate and arise by alternative splicing of a single CD46 gene. The isoforms differ in the length. CD46 cDNA encodes a signal sequence followed by four complement control protein domains (also called short consensus repeats (SCR)).