GPT and infection: Infection of U2OS-Cre cells by the VV-ΔTk-yfp-gpt-1loxP and VV-ΔTk-yfp-gpt viruses led to strong luciferase transgene expression, however expression of YFP is only detectable from cells infected with the VV-ΔTk-yfp-gpt-1loxP virus.