Researchers at the Cooperative Research Centre (CRC) for the Biological Control of Pest Animals (Canberra, Australia) have accidentally engineered a “killer virus,” wiping out all of their experimental mice. To render mice and rats sterile and ultimately decrease rodent infestations of grain crops, CRC scientists inserted a gene for interleukin-4 (IL-4) into a mousepox virus with the hope of stimulating antibodies against eggs in mice subsequently injected with the virus. However, the engineered virus became much more virulent than intended, killing all the rodents treated by suppressing cell-mediated responses against viral infection. Concerns that a similar modification of the smallpox virus could prove just as lethal to humans has renewed calls for a strengthening of the 1972 Biological and Toxins Weapons Convention (Nat. Biotechnol. 18, 806, 2000). In a 15 January letter to The Australian, CRC CEO Bob Seamark stated: “the best way to avoid misuse of this technique by unethical persons was to alert the world to it.” A report on the discovery will be published in February's Journal of Virology.