Recent advances in inducible gene expression systems.

A means of controlling the level and timing of expression of specific genes in cultured cells or in animals would have broad applications. There has been recent progress in two very promising systems: problems due to the high background expression from tetracycline-responsive promoters have been solved by constructing tetracycline-sensitive transcriptional repressors; and new rapamycin analogues have been isolated that are capable of activating the FK506-inducible system but lack the cytostatic side effects of the original inducers. Both systems now provide opportunities for expressing toxic genes, growth arrest genes, and therapeutic products in a regulated fashion previously not possible.