Stem cells and embryo-derived cell lines: tools for study of gene expression.

Embryonal carcinoma (EC) cells obtained either from teratocarcinomas or directly from in vitro cultures of mouse embryos (EK) can be used as models for the early stages of normal mammalian development. A few known examples of experimental designs with such cells are reviewed: aggregates with normal embryos, promotion of parthenogenetic development by injection of EK cells into blastocysts, EK cells homozygous for a lethal gene, timing of expression in differentiating EC cells of a tissue-specific gene product, and X chromosome inactivation.