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THE code of life embodied in DNA has been artificially extended, potentially increasing by 256 the number of different building blocks that can be incorporated into proteins. By tinkering with the cell’s natural machinery, has found a way of making proteins with entirely new properties, opening up a future of exotic designer organisms (see “Life’s code rewritten in four-letter words”). This is a fundamental advance that could lead to new drugs, materials and energy sources. But tampering with life’s operating system will inevitably raise safety concerns – and it’s true that we have no way of…