CD4 and infection: Potentially, differences in the level of expression between wild-type, chimeric and mutant Env could have an impact on the phenotypes observed; however, it is not likely that this is the case since luciferase activity in cell-to-cell fusion assays was always between 100 and 1000-fold higher than the background levels, and pseudotype infection of HOS-CD4-CCR5 cells showed limited variability between pseudotype stocks but always remained at least 1000-fold higher than background levels.