Genetically modified animal

The genetically modified animal emerged from Rudolf Jaenisch and Beatrice Mintz's 1974 experiment injecting viral DNA into mouse embryos—proving that foreign genetic material could be integrated into a mammal's germline and passed to offspring. This opened the door to animals designed for human purposes: disease models, pharmaceutical factories, and eventually the Harvard OncoMouse.

The adjacent possible required both recombinant DNA techniques and developmental biology expertise. By 1974, scientists could manipulate DNA in bacteria, but inserting genes into animals posed different challenges. Mouse embryos developed rapidly, and foreign DNA had to integrate before cells differentiated. The window was narrow.

Jaenisch and Mintz chose simian virus 40 (SV40), a DNA tumor virus whose genetic material was well characterized. Working at Fox Chase Cancer Center in Philadelphia, they injected SV40 DNA into early mouse embryos—just after fertilization but before significant development. The embryos were then implanted into surrogate mothers.

The results were remarkable: viral DNA appeared in every cell of the resulting mice, including their germ cells. Foreign genes had integrated into the host genome and would be inherited by subsequent generations. Jaenisch had created the first transgenic animal—though the term 'transgenic' wouldn't be coined until the early 1980s.

The technology advanced rapidly. In 1980, Jon Gordon and Frank Ruddle at Yale demonstrated microinjection of purified genes directly into mouse egg nuclei—a more precise technique than viral infection. Multiple university groups in the US and Europe independently developed transgenic methods, combining mouse developmental biology with genetic engineering.

The cascade transformed biomedical research. In 1983, Philip Leder and Timothy Stewart at Harvard created the OncoMouse—a mouse genetically modified to develop cancer, making it invaluable for cancer research. The OncoMouse received the first US patent for a vertebrate animal in 1988, sparking debates about patenting life that continue today.

Path dependence shaped the field. Mice became the default model organism for transgenic research due to their short generation time, known genetics, and the techniques pioneered by Jaenisch. Today, transgenic animals produce pharmaceuticals in their milk, model human diseases for drug testing, and provide organs for xenotransplantation research. Jaenisch, now at MIT, later pioneered cloning and induced pluripotent stem cells—continuing to reshape what biology can achieve.