Ever since the birth of Dolly the sheep in 1996, human cloning for reproductive purposes has seemed inevitable. Notwithstanding past dubious claims of such an achievement—including one by a company backed by a UFO cult—no human clones have been made, other than those born naturally as identical twins. Despite success with other mammals, the process has proved much more difficult in humans—which may strike some people as comforting and others as disappointing.