Measles virus (MV) causes a productive infection in humans and certain simian hosts. Rodent cells such as Chinese hamster ovary (CHO) and murine cell lines normally resist MV infection. Human CD46, or membrane cofactor protein, a complement regulatory protein, recently has been reported as the cellular receptor for MV. Multiple isoforms of the CD46 protein exist; four of these isoforms are commonly expressed on human cells. Expression of each of the four isoforms in CHO cells followed by exposure to MV led to the appearance of viral proteins within the cells and on the cell surface as detected by immunofluorescence. Syncytium formation also was observed in the cultures. CHO cells expressing any of the four isoforms and exposed to MV formed infectious centers when plated on Vero cell monolayers, indicating that the cells can transmit virus to uninfected cells. The murine cell line MC57 expressing the BC1 isoform of CD46 also stained positively for MV antigens and was positive in the infectious center assay after exposure to MV. Treatment of CD46-expressing cells with antibody to human CD46 inhibited MV binding in a dose-dependent manner. These observations indicate that any of the four primary isoforms of CD46 are able to serve as a receptor for MV.Measles virus (MV) is among the most contagious infections of humans. Despite the development and use of an effective vaccine, >1 million infants and young children die each year from measles (1). MV infects epithelial cells and cells of the immune system resulting in pneumonia, diarrhea, and profound immunosuppression leading to secondary infections. In addition, MV can cause a post-infectious hyperallergic condition, an inclusion-body encephalitis, and a persistent infection of neurons called subacute sclerosing panencephalitis (SSPE) (2). The mechanism by which MV causes immunosuppression and the pathogenesis of the acute or persistent MV infections are not well understood.MV is a member of the genus Morbillivirus in the family paramyxoviridae. It is an enveloped virus with a singlestranded RNA genome. MV contains two membrane glycoproteins, hemagglutinin (MV-H) and fusion (MV-F). Both glycoproteins are required for infection and for MVassociated syncytium formation; it is thought that MV-H is involved in virus binding to the host cell and MV-F induces fusion of viral and host cell membranes (2, 3).Expression of MV-H activates the alternative complement pathway, resulting in deposition of complement C3b protein on the surface of infected cells (4). CD46, or membrane cofactor protein, is a member of the regulators of complement activation gene cluster and functions to inhibit deposition of complement proteins C3b and C4b on host cells (5). Recently, CD46 has been reported to be the cellular receptor for MV (6, 7). Multiple isoforms of CD46 exist, of which four (C1, C2, BC1, and BC2) are commonly expressed on human cells. Certain tissues, however, may preferentially express different isoforms [e.g., kidney, salivary gland, and brain (8)]. CD46 isoforms primar...