Lack of focus formation of S+L- mink cells by the ecotropic murine leukemia viral genome.

Infection of mink S+L- cells with ecotropic murine leukemia virus, achieved by phenotypic mixing with xenotropic virus, did not result in the induction of transformed foci. Also, clonal line of S+L- mink cells, chronically infected with ecotropic murine leukemia virus, which produce both the ecotropic virus and its murine sarcoma virus pseudotype are morphologically indistinguishable from normal S+L- mink cells. Neutralizing antiserum added to S+L- mink cells inoculated with xenotropic virus 24 h earlier prevented the formation of foci of transformed cells. Together, these observations indicate that focus formation in S+L- cells requires a regional spread of infection, with the insertion of additional murine sarcoma virus genomes and resultant transformation occurring from a gene dosage effect.