Human CD46 (membrane cofactor protein) is expressed on all nucleated human cells and is a cellsurface regulator of activated complement as well as a receptor for both the measles and herpes 6 viruses. CD46 is also expressed in particularly high levels on some tumor cell lines . Complement leads to the recognition of foreign material, targeting it for removal and immune cytolysis. The primary role of CD46 is to bind to one of the complement components, C3b, thereby protecting host cells from autologous complement attack. in addition to its role as a complement regulatory factor, CD46 acts as a measles virus (MV) receptor. Despite the development and use of an effective MV vaccine, at least one million deaths occur each year. MV infection leads to downregulation of cell surface CD46 and immunosupression. MV (and probably also C3b) is internalized shortly after complex formation with CD46, yet, despite much work in this area, precisely how MV enters cells is largely unclear.