Single cell cycle infectivity of mutant and wild type virus particles on TZM-bl reporter cell lines. For relative viral infectivity assay, TZM-bl reporter cell lines were seeded one day before infection. Following day, medium was removed and target cells were inoculated by adding equal amounts of mutant and wild type NL4-3 virus produced from transfected HeLa-tat III, 293T, and COS7 cells. In this assay, expression of the reporter luciferase gene is under the control of Tat protein that is activated by Tat protein synthesized from the infecting virus. Infected cells were then analyzed 24 hrs post-infection with the luciferase assay kit obtained from Promega and as recommended by the manufacturer. RLU, relative light unit.