New research indicates that mice, if genetically altered, lose their fear of cats. Japanese scientists at the University of Tokyo say they have modified mice by removing certain nasal cells." Without the nasal cells, mice no longer smell the cat and no longer try run away--as mice are said to be naturally terrified of cats.
The scientists say that their research shows that mice are genetically designed to fear cats and that it's not learned by experience.
Of course, without the nasal cells the mice would be even more defenseless. They would approach the cat, which would then snatch them up. Cats that I've seen usually do one of the following with mice: kill then and eat them, play with them and then kill and eat them or play with them and then lose them.
Sounds to me like this gene is a survival instinct of mice. Extinction would certainly be a lot more rampant if animals suddenly had no more fear of their predators.