This seems potentially important: "Despite a 30-year lifespan that gives ample time for cells to grow cancerous, a small rodent species called a naked mole rat has never been found with tumors of any kind - and now [biologists] think they know why. ... the mole rat's cells express a gene called p16 that makes the cells 'claustrophobic,' stopping the cells' proliferation when too many of them crowd together, cutting off runaway growth before it can start. The effect of p16 is so pronounced that when researchers mutated the cells to induce a tumor, the cells' growth barely changed ... Like many animals, including humans, the mole rats have a gene called p27 that prevents cellular overcrowding, but the mole rats use another, earlier defense in gene p16. Cancer cells tend to find ways around p27, but mole rats have a double barrier that a cell must overcome before it can grow uncontrollably. ... It's very early to speculate about the implications, but if the effect of p16 can be simulated in humans we might have a way to halt cancer before it starts. ... We haven't come across this anticancer mechanism before because it doesn't exist in the two species most often used for cancer research: mice and humans. Mice are short-lived and humans are large-bodied. But this mechanism appears to exist only in small, long-lived animals."