Detection of B-tropic endogenous type-C virus in viral isolates originating from induced BALB/K-3T3 cells

Isoelectric focusing (IEF) analysis of class I endogenous type-C virus induced by iododeoxyuridine treatment of BALB/K-3T3 cells revealed, in addition to the major variant of the p30 polypeptide, which has an isoelectric point of 6.1 (pI 6.1 isop30), a minor isop30 with a pI of 5.6. This value was also found for a prototype BALB/c B-tropic endogenous virus isolate. The pI 5.6 isop30 of the N-tropic isolate was amplified by long-term virus replication in B-type mouse cells, and comparative IEF and XC-assay data suggest that it may correspond to a B-tropic subpopulation which has not yet been detected in vitro in mouse cells of embryonic origin.