Germ line chimeras from female ES cells.

Murine embryonic stem cells (ES cells) are pluripotent cells that can contribute to all tissues of developing mice including the germ line when aggregated with 8-cell-stage embryos or injected into the blastocoel cavity of murine blastocysts. As well as for in vitro studies, they are used to introduce mutations into the murine genome, thereby producing new mutant mouse lines. All ES cell lines so far described that contribute to the germ line have been male. However, for many studies female ES cell lines may be essential. Female ES cells may be particularly useful for mutating genes located on the X chromosome which would otherwise be hemizygously lethal in males. We show here that female ES cells are able to form germ line chimeras and to sex convert a male host embryo.