Vaccinia virus is a member of the Orthopoxivirus genus. Recombinant vaccinia is a very useful vector because of its stability, ability to obtain high titer stocks ( 108 to 1010 pfu/ml), and its capacity to incorporate up to 50 kb of foreign DNA in wild type vaccinia. Protein is expressed rapidly and at a high level. Vaccinia shows broad tropism in vivo and in vitro. The recombinant vaccinia virus is produced by homologous recombination between a plasmid and the viral DNA within infected cells. The gene of interest is inserted in a non-essential locus of the vaccinia DNA. To aid in the recovery of the recombinant virus, plasmids have been generated that allow for selection of the recombinant vector. Since vaccinia encodes its own DNA-dependant RNA polymerase, which does not recognize foreign promoters, the cDNA must be cloned downstream of a well defined virus promoter or close to an endogenous vaccinia promoter. Vaccinia viruses are widely used for the study of poxvirus molecular biology, functional characterization and over expression of proteins, and as candidates for live vaccines or vaccine research.