There is much interest in using farm animals as 'bioreactors' to produce large 
quantities of biopharmaceuticals. However, uncontrolled constitutive expression 
of foreign genes have been known to cause serious physiological disturbances in 
transgenic animals. The objective of this study was to test the feasibility of 
the controllable expression of an exogenous gene in the chicken. A retrovirus 
vector was designed to express GFP (green fluorescent protein) and rtTA (reverse 
tetracycline-controlled transactivator) under the control of the 
tetracycline-inducible promoter and the PGK (phosphoglycerate kinase) promoter, 
respectively. G0 founder chickens were produced by infecting the blastoderm of 
freshly laid eggs with concentrated retrovirus vector. Feeding the chickens 
obtained with doxycycline, a tetracycline derivative, resulted in emission of 
green body color under fluorescent light, and no apparent significant 
physiological dysfunctions. Successful germline transmission of the exogenous 
gene was also confirmed. Expression of the GFP gene reverted to the 
pre-induction levels when doxycycline was removed from the diet. The results 
showed that a tetracycline-inducible expression system in transgenic animals 
might be a promising solution to minimize physiological disturbances caused by 
the transgene.