Impact of the IN mutations on viral replication . (A) The late replicative steps – from viral transcription until the release of new virions in the cell supernatant- were studied by determining the reverse transcriptase (RT) activity in the culture supernatant of FAB cells transfected with equal quantities of the various proviral molecular clones. (B) To study the early replicative steps, viral infectivity was determined in a single-cycle replication assay using FAB-indicator cells . Cells were exposed to equal amounts of wild-type or IN-mutated viruses for 24 hours, as determined by RT-activity measurements in viral supernatants. Infections were assessed by measuring β-galactosidase activity in cell extracts. Data represent the mean of triplicate infections (+/- SD).