The structural and functional analyses of proteins have benefited enormously from the use of technologies of recombinant gene expression. The recombinant vaccinia virus system has been widely employed to express genes from eukaryotic, prokaryotic, and viral origins (for reviews, see refs. 2 –7 ) and several detailed protocols for the generation, identification, isolation, and characterization of recombinant vaccinia viruses have been published (8 -10 ).