Generation of completely embryonic stem cell-derived mutant mice using tetraploid blastocyst injection.

Embryonic stem (ES) cells provide a unique tool for producing specifically designed mutations in mice. Here, we describe an alternative approach toward the generation of mice which are derived completely from ES cells (ES mice), as judged by glucose phosphate isomerase (GPI) analysis, without prior passage through the germline. By injecting wild-type and mutant ES cells into tetraploid blastocysts, viable and fertile ES mice were generated, suggesting that totipotency of ES cells was not affected by long-term culture and experimental manipulation in vitro. When ES cell clones harboring a lacZ reporter gene introduced by either targeted insertion or a gene-trap approach were used, the expression pattern of the lacZ gene in ES fetuses was identical to that of fetuses that were derived from breeding of chimeric mice. Thus, this technique can be considered as a useful and rapid approach to produce fetuses and mice directly from ES cells carrying predetermined genetic changes and offers many applications for studies in molecular genetics and developmental biology.