BACKGROUND: We have previously constructed a doxycycline (dox)-dependent HIV-1 variant by incorporating the Tet-On gene regulatory system into the viral genome. Replication of this HIV-rtTA virus is driven by the dox-inducible transactivator protein rtTA, and can be switched on and off at will. We proposed this conditional-live virus as a novel vaccine approach against HIV-1. Upon vaccination, replication of HIV-rtTA can be temporarily activated by transient dox administration and controlled to the extent needed for optimal induction of the immune system. However, subsequent dox-withdrawal may impose a selection for virus variants with reduced dox-dependence. RESULTS: We simulated this on/off switching of virus replication in multiple, independent cultures and could indeed select for HIV-rtTA variants that replicated without dox. Nearly all evolved variants had acquired a typical amino acid substitution at position 56 in the rtTA protein. We developed a novel rtTA variant that blocks this undesired evolutionary route and thus prevents HIV-rtTA from losing dox-control. CONCLUSION: The loss of dox-control observed upon evolution of the dox-dependent HIV-1 variant was effectively blocked by modification of the Tet-On regulatory system.