the following components is inserted into a separate restriction site s thereof stated another way there is provided a modular assembly retroviral vector comprising the following components wherein each component of said vector is bounded by a relatively rare restriction site s such that each of said components can be independently removed and or inserted into separate restriction site s on said vector the modular assembly retroviral vectors of the invention can not only be prepared in a very straightforward manner employing a wide variety of readily available starting materials in addition the design of such vectors renders them amenable to rapid interchange of each of the retroviral components with replacement components from preexisting vectors in accordance with a still further embodiment of the present invention there are provided methods for producing transgenic animals capable of prolonged and regulated expression of heterologous gene s said method comprising introducing retroviral construct s of the invention as described hereinabove into early stage embryos or stem cells as used herein the phrase 34 transgenic animal 34 refers to an animal that contains one or more inheritable expression constructs containing one or more heterologous gene s under the transcription control of an operator or hormone response element as described herein optionally an invention transgenic animal also contains one or more inheritable expression constructs containing a member of the steroid thyroid superfamily of receptors that functions as a silent partner for the active receptor subunit e g rxr as employed herein 34 introducing 34 embraces a variety of techniques for contacting target cells with retroviral construct s of the invention.