Abstract
The Fv-1 murine gene controls predominantly the replication of leukemia viruses of murine cells. Forced passage by B-tropic Friend leukemia virus in the restrictive host cells (NIH, Fv-1n/n) results in viral progeny capable of replicating efficiently in murine cells of any Fv-1 type, which are denoted as NB-tropic virus. We have studied the RNase T1-resistant oligonucleotide pattern of a series of NB-tropic Friend virus isolates and have been able to show changes from the parental B-tropic virus which occur at the 5' end of the genome. Cloned NB-tropic virus falls into three classes, demonstrating either four, one, or no apparent changes in the genome. These results suggest the possibility that conversion to NB tropism occurs by a recombination mechanism but, since change to NB tropism can occur without any observable oligonucleotide alteration, they do not confirm that any single oligonucleotide is diagnostic of NB tropism.