Genetically modified fish could soon be on the table1 THE Belgian blue is an ugly but tasty cow that has 40% more muscle than it should have. It is the product of random mutation followed by selective breeding—as, indeed, are all domesticated creatures. But where an old art has led, a new one may follow. By understanding which genetic changes have been consolidated in the Belgian blue, it may be possible to design and build similar versions of other species using genetic engineering as a short-cut. And that is precisely what TerryBradley, a fish biologist at the University of Rhode Island, is trying to do. Instead of cattle, he is doing it in trout. His is one of two projects that may soon put the first biotech animals on the dinner table.2 Belgian blues are so big because their genes for a protein called myostatin do not work properly. Myostatin is a hormone that regulates muscle growth. Disable its action and muscles will grow in parts of the anatomy where other animals do not even have them.3 Dr Bradley has launched a four-pronged attack on the myostatin(肌肉生长抑制素) in his trout. [First], he has introduced into them a gene that turns out a stunted version of the myostatin receptor, the molecule that sits in the surface membrane of muscle cells and receives the message to stop growing. The stunted receptor does not pass the message on properly.4 He has [also] added two genes for non-functional variants of myostatin. These churn out proteins, and those proteins bind to the receptors, but they do not tweak them in a way that passes the message on. They do, however, swamp and dilute the effect of functional myostatin molecules.5 [Finally], he has spliced in a gene that causes overproduction of another protein, follistatin. This binds to myostatin and renders it inoperative.6 The upshot of all this tinkering is a trout that has twice the overall muscle mass of its traditional counterparts. Moreover, this muscle is low in fat, like that of its bovine counterparts. That, and the fact that the animal’s other organs are unaffected, means it does not take twice as much food to grow a fish to maturity.7 The genetic engineers at Aqua Bounty, a company based in Waltham, Massachusetts, have taken a different route using a different species. They are trying to grow supersize salmon by tinkering with the genes for growth hormone(生长激素).