SO let's recap: what's all the fuss about? Genetically modified, genetically engineered, genetically altered - the terms all mean putting the DNA from one living thing into something else. It can mean your tomato has fish genes in it. Genes are the long molecules in your cells that direct who you are - your eye colour, your height. Say you want a huge tomato. You could, say, replace the puny tomato-growth gene with a fish-growing gene and you get a salmon-sized tomato. So what? Sounds great to me. It could be, and the many companies doing it - Monsanto in the US, Novartis in Britain, for instance - insist that it is. But it's not that simple. First, you're mixing genes that otherwise would never get mixed. What happens to your tomatoes when they're all salmon-sized? Will they catch unexpected diseases and die, so growers lose a whole crop? This is a worry for farmers in poor countries. Then there is the issue of whether the genes can creep out of your greenhouse tomatoes and into something else - say, the fast-growing water hyacinth - what you see all over the water at Macau. Do we want water hyacinth blocking all the rivers? Or a gene for resistance to weed-killer getting into weeds? The companies use crippled viruses to carry the DNA where they want it to go. What if they become uncrippled? Anyway, do you want to eat tomatoes with fish in them? You've not been asked and are not being given a choice. If food is labelled, you can choose. Surely they've tested all this. And from my limited knowledge, when you eat it, the gene will just break up in your stomach. They have tested, a lot. And the US regulatory agency, the Food and Drug Administration, agrees with the companies that most GM foods on sale so far are so similar to their unaltered varieties that there's no substantial difference. But the companies have done the tests, in a controlled setting for a short time. And they are keen to get their products to the shops. Critics such as Greenpeace say the tests are too short and restricted, and cannot tell us the effects on humans over generations. They say the firms have told governments how to set the tests. Many other research studies are producing alarming results. One was published recently where two researchers at Purdue University in the US were looking at fish that had been given human growth hormone genes to make them grow bigger, quicker. Salmon growers worldwide are experimenting with this. The pair found that the gene quickly spread through all the fish population, but then only two-thirds of them lived to reproduce. So within a few generations, your fish are extinct. As for eating the genes, it's true they won't harm you. But the genes are there to produce proteins that will. Anyone allergic to nuts won't be keen to eat anything containing a nut gene. Many of the genes they're considering using we wouldn't normally eat, so the effects are unknown. But the companies say the new foods will help solve world food shortages by improving productivity, and they'll be able to do wonderful things such as put hepatitis vaccines into bananas so children in, say, Bangladesh can eat one and be vaccinated. No needles, no problems with poorly trained nurses in isolated places. They do say that. But there's enough food in the world already, it's a distribution and a political problem that stops the food getting to the people who need it. As for vaccines in bananas, that would be marvellous. Professor Samuel Sun Sai-ming of Chinese University is working on improving things such as the nutrition of Chinese rice, which is great. But the companies haven't produced those - they've only made things such as pesticide-resistant soyabean, which helps their profits but not consumers. Perhaps if they had started with foods that helped the public, they would have been more welcomed than they have been.