Ecotropic recombinant virus (ERV), a relatively new class of murine retrovirus endogenous to mice, is expressed at significant levels by most murine myeloma and hybridoma cells examined. The routine XC, S+L-, mink cell focus-inducing (MCF), and reverse transcriptase (RT) tests are not suitable to detect and quantify the levels of ERV. A serological focus assay, based on specific anti-murine leukemia virus (MuLV) viral envelope (env) antibodies, is required to detect ERV. A more sensitive format of this serological focus assay includes co-cultivation of test article cells with the indicator (Mus dunni) cells. ERV isolated from murine hybridoma cells show a unique pattern of cross-reactivity with anti-MuLV env antibodies and this pattern is clearly distinct from that of ectropic and xenotropic retroviruses.
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