Hwang and his colleagues claimed to have replaced the nucleus of an unfertilized human egg with the nucleus of an ordinary cell taken from another person, developing stem-cell lines from the resulting embryo that matched the DNA of the person who supplied the ordinary cell. That achievement appeared to take us significantly closer to a world in which patients could be given cell or tissue transplants that their bodies would not reject, because the biological materials, cloned from the patients themselves, would be a perfect match.