Provided is a gene expression control method that employs a transcription termination sequence positioned within an intron. The transcription termination sequence is disruptable by the addition of a trans-acting factor. For example, in a "dual splicing switch", the transcription termination sequence is flanked by recombination sites and can be excised by a recombinase. The Cre/LoxP recombination system may be used for this purpose. In use, the intron containing the disruptable transcription termination sequence is positioned within a reading frame of a target gene. Also provided is a nucleic acid containing this transcription control mechanism and a cell line harboring a gene containing the nucleic acid. In one example, the transcription control mechanism is used to produce a cell line containing a toxic gene, the AAV Rep gene. A cell line for producing recombinant AAV virus particles, or recombinant parvovirus particles, also is provided along with a method for producing the same.
