Ecdysone is the steroidal prohormone of the major insect moulting hormone 20-hydroxyecdysone. It groups with its homologues the steroidal molting hormones in arthropods, but they also occur in other phyla (Invertebrates and Tunicates) where they can play different roles. Ecdysone receptor (EcR), functions as a receptor for the ecdysteroid molting hormones of insects. Since ecdysone administration has no apparent effect on mammals, its use for regulating genes should be excellent for transient inducible expression of any gene in transgenic mice and for gene therapy.