Human or rat growth hormone (GH) genes have been introduced into all cells of a mouse by microinjection of fertilized eggs but they were not expressed under their own promoters. However, substitution of a mouse metallothionein (MT) promoter allowed expression and regulation comparable to that of the endogenous MT genes. These fusion genes have been used to stimulate the growth of both normal mice and dwarf mice that lack sufficient GH. Substitution of a rat elastase-I promoter directed expression of GH exclusively to the acinar cells of the pancreas. Progress has been made towards developing the hGH gene into a vector that is not expressed in vivo unless an enhancer element is inserted. Recombination between overlapping DNA fragments derived from a MThGH gene, each of which is nonfunctional, has been observed when they are coinjected into mouse eggs. In some cases, functional hGH was produced as evidenced by enhanced growth of the mice.