The present invention relates to RNA vectors comprising alphavirus RNA havinginserted therein a recombinant retrovirus genome containing a foreign RNAsequence encoding a biologically active substance. The vectors of the presentinvention provide for the high level expression of the recombinant retrovirusgenome directly in the cell cytoplasm of eukaryotic cells. When coexpressedwith structural proteins of the retrovirus, the recombinant genome canefficiently be packaged into infectious recombinant retrovirus, also calledretrovirus vectors, which can transduce the foreign RNA into recipient cells.Most importantly this cytoplasmic expression system facilitates the efficientproduction of vectors containing foreign RNA comprising genes in combinationwith introns or other control elements of gene expression. Such vectors havebeen impossible or very difficult to produce by the conventional nuclearexpression systems because of RNA splicing.
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