Schematic representation of the construction of the chimeric vaccinia viruses and structure of the plasmid pR-XSNegfp. (A) In step 1, the retroviral gag-pol sequences were inserted into the vaccinia virus hemagglutinin locus (HA locus). The lacZ and gpt markers were used transiently and are not present in the first construct, the virus vHA-MLVg. The second step was insertion of the VSV-G pseudo-env gene into the D4/D5 intergenic region (D4/D5 locus), resulting in the packaging virus. The last step consisted of insertion of a retroviral vector unit into the packaging virus, resulting in the triple virus. (B) Structure of the triple virus. (C) Structure of plasmid pR-XSNegfp. This plasmid transfers the retroviral vector genome together with a permanent gpt marker into the vaccinia virus thymidine kinase (tk) locus; PR, synthetic vaccinia virus early promoter R; U5, U5 region of the retroviral long terminal repeat; Ψ, packaging signal; EGFP, open reading frame of the EGFP gene; SV40, simian virus 40 early promoter; neo, neomycin resistance gene; 3′LTR, 3′ retroviral long terminal repeat; P7.5, vaccinia virus early/late promoter P7.5; gpt, selection marker gene encoding the Escherichia coli enzyme guanine phosphoribosyltransferase.