Abstract: An inducible viral replicon comprising at least one inducible repressor and/or activator, and all viral sequences which are essential for replication under direct or indirect control of said inducible repressor and/or activator. The inducible repressor and/or activator may comprise a Tet operon. A replicon of the invention may be used to produce a virus, for instance an attenuated HIV virus, which can be induced to replicate by the presence of doxycycline or a functional analog thereof. The replicon and/or the produced virus may be used to prepare a vaccine, for instance for the prophylaxis of AIDS. A replicon of the invention can be modified, preferably improved, by culturing the replicon in permissive cells for an extended period of time.