Because transgenesis via embryonic stem (ES) cells has become so topical in recent years, there have been a plethora of technical accounts of how to produce chimeras by introducing these cells into the preimplantation mouse conceptus, both using and avoiding micromanipulation (1 –5 ). In the overwhelming majority of cases this type of chimerism is regarded simply as a means to an end, namely establishing and perpetuating in vivo modifications to the genome that have been produced in vitro. However, it is also being exploited to a limited extent to assess the developmental potential of homozygous mutant ES cells in wild-type embryos and vice versa (6 ,7 ).
