Measles virus (MV) enters cells either through the signaling lymphocyte activation molecule SLAM (CD150)expressed only in immune cells or through the ubiquitously expressed regulator of complement activation, CD46. To identify residues on the attachment protein hemagglutinin (H) essential for fusion support through either receptor, we devised a strategy based on analysis of morbillivirus H-protein sequences, iterative cycles of mutant protein production followed by receptor-based functional assays, and a novel MV H three-dimensional model. This model uses the Newcastle disease virus hemagglutinin-neuraminidase protein structure as a template. We identified seven amino acids important for SLAM-and nine for CD46 (Vero cell receptor)-induced fusion. The MV H three-dimensional model suggests (i) that SLAM-and CD46-relevant residues are located in contiguous areas in propeller -sheets 5 and 4, respectively; (ii) that two clusters of SLAM-relevant residues exist and that they are accessible for receptor contact; and (iii) that several CD46-relevant amino acids may be shielded from direct receptor contacts. It appears likely that certain residues support receptorspecific H-protein conformational changes. To verify the importance of the H residues identified with the cell-cell fusion assays for virus entry into cells, we transferred the relevant mutations into genomic MV cDNAs. Indeed, we were able to recover recombinant viruses, and we showed that these replicate selectively in cells expressing SLAM or CD46. Selectively receptor-blind viruses will be used to study MV pathogenesis and may have applications for the production of novel vaccines and therapeutics.Measles, caused by wild-type measles viruses (MVs), is one of the leading causes of infant death in developing countries (11). The immune suppression that accompanies measles significantly enhances an individual's susceptibility to secondary infections, and these infections account for most of the morbidity and mortality (4). Vaccination with the live attenuated strain Edmonston (MV-Edm) prevents measles-related fatalities and only rarely results in the development of mild symptoms (17). Cell entry likely plays a central role in MV pathology: most wild-type MV strains preferentially use the immunecell-specific protein SLAM as a receptor (19,27,51,80), whereas MV-Edm enters cells more efficiently using the ubiquitous protein CD46 (16,47,72). Since the three morbilliviruses-MV, canine distemper virus (CDV), and rinderpest virus (RV)-enter cells through SLAM (human, canine, or bovine) and are immunosuppressive (81), SLAM-dependent cell entry may be directly related to pathogenesis; CD46 interactions have also been correlated with immunosuppression (32,37,50).The attachment protein of morbilliviruses has hemagglutination but not neuraminidase activity and is therefore named hemagglutinin (H) rather than HN (hemagglutinin-neuraminidase). H is a type II transmembrane glycoprotein that dimerizes in the endoplasmic reticulum (58, 67). After binding to the receptor, it sup...