[maratc]

> If you have 100 mice, it's quite likely that around 20 of them will die within one year of cancer.

If you have 100 people, about 40 of them will be diagnosed with cancer over their lifetime (39/100 females and 41/100 males). Note "diagnosed with" is very different from "die" and "over lifetime" is not the same as "within one year", but the probability of people getting cancer naturally is high as well.

[cyberax]

Sure. But in general, cancer risk should scale with the number of cell divisions, which is fairly low for mice. And other small, short-lived mammals like weasels are not as susceptible to cancer. Naked mole rats (also rodents!) are downright cancer-proof.

This all means that mice don't have a good natural cancer-suppression machinery, and this in turn makes them somewhat awkward to use to discover new treatments.