The first transgenic mice resulted from microinjecting fertilized eggs with vector DNA similar to that diagrammed in Figure 16.15 except that it contained a promoter for the mammalian metallothionein gene linked to the HGH gene. The resulting transgenic mice showed elevated levels of HGH in tissues of organs other than the pituitary gland—for example, in heart, lung, and liver—and the pituitary gland underwent atrophy. How might the production of HGH in transgenic animals be better regulated, with expression restricted to the pituitary gland?