Gene-drive technology has been compared to Kurt Vonnegut’s ice-nine, a single shard of which is enough to freeze all the water in the world. A single X-shredder mouse on the loose could, it’s feared, have a similarly chilling effect—a sort of mice-nine. To guard against a Vonnegutian catastrophe, various fail-safe schemes have been proposed, with names like “killer-rescue,” “multi-locus assortment,” and “daisy-chain.” All of them share a basic, hopeful premise: that it should be possible to engineer a gene drive that’s effective and at the same time not too effective. Such a drive might be
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