THAT murine embryonic stem (ES) cells can be used as a bridge between genetic
manipulations in vitro and biological analysis in vivo is one ofthe most significant findings of
the last decade. Thisunique property is made possible because ES cells can be maintained
as a stable diploid population in vitro and upon their reintroduction into host embryos, they
can also colonize all fetal tissues, including the germline. Furthermore, ES cells can be
stably transfected and subsequently offer a route to transgenesis following extensive in vitro …