Wild-type vaccinia virus strains WR and IHDW were used as controls for the studies of the temperature mutants Cts52, Dts41 and Dts80 (Kato et al., 2008). Throughout this work, viruses were grown in BSC40 cells, an African green monkey cell line. The methodology for cell culture, virus infection, purification, plaque assay and one-step growth have been described before (Condit and Motyczka, 1981; Condit et al., 1983; Kato et al., 2004). The non-permissive temperature for analysis of ts mutants was 39.7°C but in the text is designated as 40°C for simplicity.