Thousands of experiments are conducted worldwide each year that involve genetically modifying organisms and viruses. Genetic engineering of model organisms genomes, usually to alter the expression of a protein, is an essential tool for researchers to investigate the in vivo function of genes and their regulation. With so many manipulations happening at any given time, it can be expected that some of them will generate unexpected results or even ones with potential dual-use implications.

One such experiment published in the Journal of Virology shortly before the 2001 anthrax attacks was a lightning rod for concerns over how genetic engineering may contribute to bioweapons development. In an attempt to create a contraceptive vaccine for mice as means of pest control, Drs. Ron Jackson and Ian Ramshaw used the mousepox virus to express high levels of an egg protein in female mice to trigger an immune response. The idea is simple; if enough egg protein is expressed, then female mice will develop an immune response to their own eggs and become sterile. To begin with, the researchers inserted the gene for interleukin-4 (IL-4) into the mousepox virus to maximize the formation of antibodies. Surprisingly, the virus expressing IL-4 suppressed the normal immune response of mice, killing most of them, including those vaccinated against the mousepox virus. Since mousepox is a member of the same family of viruses as smallpox, the results clearly demonstrated that genetic engineering has the potential to render existing smallpox vaccines useless.