The ability to introduce foreign genes into the mammalian germ line has proved to be one of
the most powerful experimental tools in mammalian developmental genetics. This
technology arose from the knowledge, attained through gene transfer experiments in tissue
culture, that new DNA could be inserted into the genome of a host cell and subsequently
efficiently expressed (reviewed by Scangos and Ruddle~). The key unknown factors at the
time efforts were begun to insert DNA into an intact mammal related to the frequency of gene …