On Apr. 12, 1988, OncoMouse became the first animal to be patented in the United States (U.S. Patent 4,736,866). OncoMice are genetically modified to have an active cancer gene, making them very likely to develop cancer. Scientists hoped this trait would make the mice useful test subjects for cancer research.

In early 1983 Harvard University scientists Philip Leder and Timothy Stewart created OncoMice by using a fine glass needle to inject known cancer genes into mouse embryos just after fertilization. This genetic modification not only made the mice prone to cancer, but also ensured that they would pass the cancer genes to their offspring.

American chemical company DuPont funded much of this research. As a result, Harvard gave DuPont priority to license the patent, making the company the sole distributor of OncoMice.