Extinctions are seldom cause for celebration. Humans are wiping out species at a frightening rate, whether hunting them into history or, far more threateningly, damaging the habitats on which they depend. But occasionally, the destruction is warranted. Smallpox was officially eradicated in 1980, and no one laments the fate of the virus that caused it; campaigns to save the virus that causes polio are thin on the ground. How, then, to think about a new technology that will make driving a species to extinction far easier?