Often drug studies, such as in the case of Fialuridine. Showed excellent results in animal studies (including mice, rats, dogs, and primates) with no ill effect

Often drug studies, such as in the case of Fialuridine. Showed excellent results in animal studies (including mice, rats, dogs, and primates) with no ill effects, but ended up being fatally toxic for humans. Turned out it was toxic in humans because a specific protein on the surface of our mitochondria would move it inside our mitochondria, where it would poison/destroy our mitochondria. This didn't happen in the animal studies (even though they have the same protein), because 3 DNA mutations caused the gene encoding the protein to be just different enough, that it would prevent it from being able to transport the drug into their cell’s mitochondria. Now that said, just because one promising animal study failed to work for humans, doesn't mean they all will fail. A lot of animal studies do in fact also hold for humans. It's just that, one shouldn't jump to conclusions either which way.