Two assays were developed for identifying individual cells which bear murine leukemia virus receptors: an erythrocyte rosette assay for ecotropic receptors, and an efficient immune cytotoxic assay for cells with ecotropic or amphotropic receptors. Both assays indicate that ecotropic MuLV adsorbed to its cell surface receptor only slowly becomes internalized. Furthermore, attempts to isolate murine fibroblast variants lacking these ecotropic MuLV receptors were unsuccessful, suggesting either that mutations in the receptor gene are rare (less than 10' per generation) or that the receptor is required for cell viability. These assays are rapid and can be used to identify receptor-bearing cells in mixed populations, a prerequisite for molecular genetic studies.
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