Cells and chromosomes of the embryo can be labeled by integrating foreign DNA into its genome. Genes coding for some cytotoxic products can selectively destroy embryonic rudiments in which these genes are expressed. Insertional mutations arise in transgenic animals and this facilitates identification and cloning of the genes that affect the development. Embryos carrying mutations in certain genes were produced using transformed stem cells. Some kinds of mutations can be corrected by introducing the cloned genes into mutant oocytes. Use of transgenic animals substantially increases the properties of methods employed in developmental genetics and experimental embryology of mammals.