(I) Since the cloned-sheep Dolly's birth, researchers have cloned many other mammals, including mice, cats, horses, cows and pigs. (II) Such reproductive cloning has many potential applications. (III) Their most advanced embryo, however, had stopped growing at about six cells, and their intention was solely to harvest embryonic stem cells. (IV) By using genetically engineered donor nuclei, geneticists can study the effects of changing single genes or combinations of genes. (V) And in the future, biologists may routinely produce genetically identical animals for experimentation, a potential benefit to genetics research.