Replication kinetics of NL4-3, NLΔSL1, and revertants. (A) Changes in the infectivity of NLΔSL1 were observed. PM-1 cells were infected with p24-normalized NL4-3 or NLΔSL1. Virus production was measured in TZM-bl cells using culture supernatant from the infected PM-1 at different times. (B) NLΔSL1 revertants were replication competent in PM-1 cells. Culture supernatants from day 14 p.i. with NLΔSL1#1 (NLΔSL1-D14) and day 22 p.i. with NLΔSL#2 (NLΔSL1-D22) were normalized to p24 and used to infect fresh PM-1 cells, and virus production was detected as described previously.