76 R. Weppelman
Integration into the germ line was not tested and probably did not occur
given the advanced age of the embryos when infected.
 The retroviral approach to producing transgenic animals is clearly effec-
tive and in many ways more convenient than microinjection. However,
any large-scale application of the retroviral approach to producing
transgenic stock animals would result in the dissemination of genes from
viruses which as a group are associated with such diseases as AIDS and
cancer and will certainly be deferred until legitimate safety concerns have
been addressed.
 A potential technical problem with transgenic animals arises from the
possibility that expression of a particular gene might be very desirable
during one phase of an animal's life but very damaging during another. As
an example, female mice transgenic for the growth hormone gene are usu-
ally infertile (Hammer et al., 1985a). This general problem doubtlessly
will be solved by including regulatory elements in the gene which put the
gene under the control of the animal's own regulatory systems or which
enable the gene to be turned on by simply adding an inducer to the ani-
mal's diet. An example of the latter is the growth hormone gene construct
used by Palmiter and Brinster to produce transgenic mice. In this construct
the growth hormone gene was placed under the control of the regulatory
element for the metallothionine gene. Because the metallothionine gene is
normally induced by zinc, growth hormone expression in the transgenic
mice could be regulated by simply adjusting the level of zinc in their diet.


 SUMMARY


THE PRESENT

 Biotechnology has the potential to supply the animal industry with im-
proved and novel diagnostics, vaccines, production improvers, therapeu-
tics and even stock animals. In the case of diagnostics and vaccines, this
potential has been partially realized. In the case of the growth hormones as
production improvers, realization awaits only the development of appro-
priate supporting technology by formulation scientists. And in the cases of
transgenic animals and therapeutics like the interferons and interleukins,
more research is needed to define problems and potential.