Abstract: A transgenic animal is constructed in which one or more cells contain a promoter to the gene for cytosolic PEPCK operably linked to a non-PEPCK gene of interest. Expression of this gene is controlled by modifying the protein and carbohydrate components of the animal's diet, or by direct hormonal regulation. The PEPCK promoter is induced by high protein and inhibited by high carbohydrate, or more directly by cAMP and insulin. The linked gene is expressed essentially only after bith and essentially only in particular tissues. The PEPCK promoter has a extremely high promoter strength.

Abstract: This invention is directed to a method for the preparation of carrier cells capable of delivering exogenous genetic material to a particular tissue of the body by means of embryonic cells competent to develop into that tissue, and essentially only that tissue, said cells bearing the exogenous genetic material. The preferred carrier cells are mesodermal cells of the yolk sac or embryonic forebrain or midbrain cells, and the desired genetic material is preferably introduced into the cells by in vitro transformation with an amphotrophic retroviral vector.