Eau de feline normally sends mice scurrying, but not so in a new breed of mouse engineered by Japanese researchers to lack a subset of olfactory neurons in the nose. The resulting rodents sidled up to their mortal enemy for a leisurely how-do-you-do [above], implying that a mouse's "fear" of cats is innate, not learned, the team reports in a recent issue of Nature. Smell-based learning was intact in the mice, however: with training, they avoided cats like any self-respecting rodent.