Summary
Recombinant vaccinia viruses based on the highly attenuated Modified Vaccinia Ankara (MVA) strain expressing HIV-1 antigen genes were constructed by a novel procedure involving the transient use of two marker genes. The selectable markers used, the Escherichia coli guanine phosphoribosyltransferase (gpt) and the ß-galactosidase (lacZ) genes, are not retained within the final recombinant virus. The transient marker stabilisation (TMS) procedure allows the generation of marker-free recombinant viruses in a series of simple plaque purification steps. HIV-l gag pol genes were inserted into two loci of vaccinia MVA demonstrating the efficiency of the procedure.