Abstract: Discoveries are disclosed that show that certain mutations in different parts of the mechanism for regulation of independently replicating element replication can be combined in one expression independently replicating element to produce a runaway-replication phenotype that is suppressible by a diffusible factor from another independently replicating element co-resident in the host cell. According to the present invention, an expression independently replicating element combining known inducible promoters with this runaway-replication phenotype is used in combination with a independently replicating element that suppresses this runaway phenotype to establish a gene expression system that provides both controllable gene amplification and controllable induction of gene expression without the use of chemical inducers or temperature shifts. This expression system produces high yields of proteins in readily isolatable forms.