Replication-defective ecotropic murine leukemia viruses: Detection and quantitation of infectivity using helper-dependent XC plaque formation.

Clones 8A and NP-N, which appear to be infected with replication-defective variants of murine leukemia virus, produce particles which do not form plques in the XC test. These particles formed XC plaques when amphotropic murine leukemia virus, which is XC negative, was added to the assay plates. This phenomenon can be used as a quantitiative infectivity assay for these replication-defective murine leukemia viruses.