into cultured ES cells, and these can be reintroduced into the ICM of an intact embryo. The result is a chimeric mouse, which develops with a portion of its cells containing the ES cell genome. The aim of such a procedure is to incorporate the mutated gene into the germ line of the mouse such that its progeny will be missing one or both alleles of the gene of interest. Geneticists widely take advantage of this ICM manipulation technique in studying the function of genes in the mammalian system. Additional images See also Homeobox genes References Developmental biology