Transient dominant selection of recombinant vaccinia viruses

A general method for constructing and selecting recombinant vaccinia viruses with insertions, deletions, or mutations in any gene that is similar in principle to one originally devised for Saccharomyces cerevisiae (S. Scherer and R. W. Davis, Proc. Natl. Acad. Sci. USA 76:4951-4955, 1979) is described. The selectable marker used, Escherichia coli guanine phosphoribosyltransferase, is not retained within the final recombinant virus, and hence, this procedure may be used serially to introduce several foreign genes or to make multiple site-directed mutations.