Bruce Ames in the articles on Cancer causation listed on the bibliography on
my site talks about how mice and rats react very differently to some
carcinogens.  He explains this by saying that rats and mice eat far
different diets in Nature.  He further explains that extrapolating human
results from the animal tests is a very inexact science because of the far
greated genetic differences between rodents and people.

In the same series of articles, he makes the point that any organic molecule
that would not be found in a hunter-gatherer's diet is a potential human
carcinogen.

Ray Audette
Author "NeanderThin"
http://www.neanderthin.com