years. Genet ic surgery - the ability to snip out pieces of DNA and move them to new places - has done remarkable things, but so far has done little to cure disease . It might, though , help prevent the world's population from starving, at least according to enthusiasts for genetically modified (GM) foods. They may be right. It has proved remarkably easy to move plant genes around. Already there are crops that have been altered to make them resistant to parasites, or to artificial weedkillers (which means that the fields can be sprayed, leaving the crop unharmed) . Commercial optimism has, in Europe if not the United States, been matched by public concerns about health risks. Why people are worried by the remote risk that GM foods might be dangerous to eat when they are happy to eat cheeseburgers that definitely are, mystifies scientists, but science is less important than what consumers are willing to accept. Unless attitudes change, the hope of putting genes for, say, essential nutrients into Third World crops will probably not be fulfilled. If interfering with plants alarms society, to do the same with animals outrages a vocal part of it. We still know rather little about how a fertilized egg turns into an adult, with hundreds of different kinds of tissue, each bearing exactly the same genetic message but with jobs as different as brain cells and bone. Although it has long been possible to grow adult plants and even frogs from single cells, the notion that it might be possible to do so with mammals seemed a fantasy - until the birth of Dolly the sheep in 1997. Then, with the simple trick of inserting the nucleus from an adult cell into an emptied egg and allowing it to develop inside a foster-mother, a sheep was made without sex: it was cloned. Cloned sheep or cows might be important in farming, and might be used to make multiple copies of animals with inserted human genes for proteins such as growth hormone (which are already used in "pharming", the production of valuable drugs in milk). The publicity that followed Dolly led to immediate condemnation of the idea of human cloning, often without much thought as to quite why it should be so horrific . After all, we are used to identical twins (who are clones of each other) , so why should an artificial version cause such horror? In the end, again, public opinion moulds what science can do, and the prospect of cloning a human being seems remote . And why might anyone want to do it? Claims of an army of identical Saddam Husseins verge on the silly, and others of replicating a loved child who died young also seem unlikely . However, the technique has great promise in medicine. Cells of the very early embryo (stem cells, as they are called) have the potential to divide into a variety of tissues, and can be grown - cloned - in the laboratory, or even manipulated with foreign genes . Perhaps they could make new skin or blood cells, or, in time, even whole organs. Because this involves the use of very early embryos, made perhaps by artificial fertilization in the laboratory and not needed for implantation into 174