Blocking the loss of dox-control by the triple safety-lock mutations. SupT1 cells were transfected with HIV-rtTA containing the triple safety-lock mutations (HIV-rtTAG19F E37L P56K) at 1000 ng/ml dox and split into 24 independent cultures (different symbols represent different cultures). At day 3, dox was washed out and the cultures were continued with dox-free medium. At day 60, all cultures were split in two parts, and dox (1000 ng/ml) was added to one of the samples. Virus production was monitored by CA-p24 ELISA on culture supernatant samples.