I wonder if they could produce rats WITHOUT cancer - turn off the genes that cause them to develop tumors. I know a number of people who breed domestic rats as pets and would love to introduce cancer-free individuals into their breeding lines, even if it was very expensive. The sad truth is that nearly 100% of the rats you can buy at a pet store will develop cancer at some point in their lives due to the willy-nilly way they're bred. Even well-bred rats from careful breeders have a good chance of developing a tumor because it's difficult to eliminate the genes (females especially don't usually develop tumors until after they're past breeding age, when it's too late).
They are remarkable little critters overall. I would still be keeping them if it weren't so heartbreaking to have them explode with cancer before the age of 2.