Naked mole rats have an uncanny resistance to cancer—so much so that a single natural case had never been observed in the long-lived creatures. But that disease-free run may finally be coming to an end, The Scientist reports. A study published last week in the journal Veterinary Pathology describes two zoo-housed rats—one outside of Chicago and one in Washington, D.C.—with two different forms of cancer. One of the rats had to be euthanized because of skin and weight problems, likely caused by its tumor. The discovery may change the way researchers use the rodents for cancer research. Until now, naked mole rats have been a standard model for studying cancer, yielding discoveries about everything from proteins that stop cells from crowding together to a potential tumor-busting sugar.