CD46 is a transmembrane protein also known as complement membrane cofactor protein, MCP, and measles virus receptor. It is widely expressed on leukocytes, platelets, epithelial cells, and fibroblasts. Multiple isoforms of CD46 have been reported with molecular weights ranging from 45-75 kD. CD46 binds complement components C3b and C4b and has been reported to play a role in T cell regulation. In addition to the complement components, CD46 has been shown to interact with moesin, c-Src, tetraspanin 4, c-Yes, and integrin beta1. CD46 is also known to be a cellular receptor for human measles virus and human herpes virus-6 as well as other human pathogens, such as Streptococcus pyogenes. Defects in CD46 have been associated with hemolytic-uremic syndrome.