Novel viral polynucleotide sequences and virus preparations recovered therefrom are provided for use as vaccines. In viruses having long terminal repeats (LTRs). where the LTRs includetranscriptional regulatory functions, at least one of the LTR regions is modified to substantially decrease the activity of at least one locus involved in transcriptional regulation. Specifically, feline leukemia virus (FeLV) is modified in the LTR by deleting at least a portion of an enhancer region and the resulting modified FeLV DNA and modified virus derived therefrom shown to be infectious and capable of inducing an immune response in a mammalian host without the serious pathological symptoms normally associated with FeLV. E. coli strain HB101 (pRF3X) was deposited at the American Type Culture Collection on October 14, 1983 and granted accession no. 39470.
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