Our system utilized two cell lines: HeLa T4+ and HEK293T. HeLa T4+ cells expressing CD4 receptor and CXCR4 co-receptor were transfected with the LTR-driven copGFP reporter gene whereas HEK293T cells were co-transfected with plasmids to express viral Env and Tat proteins. Although syncytial formations could be observed by 24 hours, copGFP readout was best to be measured on day 4 after co-cultivation.