Scientists have discovered, quite by accident, a mutant mouse strain that is capable of fighting off cancer, according to a new report. The results of a study published online this week by the Proceedings of the National Academy of Sciences indicate that the animals can survive numerous injections of cancer cells without succumbing to the disease.

During a routine study led by Zheng Cui at the Comprehensive Cancer Center of Wake Forest University, researchers administered a virulent type of cancer cell to a variety of strains of laboratory rats and mice. Typically, these injections lead to tumor growth in the abdomen within two weeks, and the cancer subsequently spreads to other vital organs. One male mouse, however, did not show any signs of tumor growth despite repeated injections. When the mouse bred with a normal female, his offspring retained the ability to fight off cancer. This ability has been passed down through seven generations so far, and there are currently 700 of the cancer-fighting animals, which are normal in every other way. Among these animals, their abilities to avoid cancer vary slightly. Some never develop any signs of the disease after an injection of cancer cells, whereas others initially show signs of cancerous growth that later spontaneously recedes. Cui notes that after multiple injections of cancerous cells, "the mice became healthy and immediately resumed normal activities, including mating."

The immune systems of the newly discovered animals are able to attack cancerous cells, but leave normal cells alone. (In the image above, the larger cell on the bottom is cancerous, and the three cells attacking it are immune cells.) This is an improvement on previous mouse models that launched an immune-system attack on cancer, because those animals went on to develop autoimmune diseases. Cui comments that the new results "suggest a previously unrecognized mechanism by which the body can fight off cancer." Of course, humans are very different than rodents. But study co-author Mark Willingham of Wake Forest remarks that the findings "are at a preliminary stage but very promising. Our hope is that, someday, this will have an impact on human cancer."