To engineer JX-594, human GMCSF gene (encoding granulocyte macrophage colony-stimulating factor or GM-CSF; driven by a synthetic early/late promoter) and lacZ gene (encoding β-galactosidase or β-gal; driven by the p7.5 early/late promoter) were inserted into the TK gene (encoding thymidine kinase or TK) in the J segment of the Wyeth strain vaccinia virus.