Rudolf Jaenisch (1942- ) is a German pioneer of transgenic science, in which an animal’s genetic makeup is altered. Jaenisch has focused on creating transgenic mice to study cancer and neurological diseases.

Jaenisch’s first breakthrough occurred in 1974 when he showed that foreign DNA could be integrated into the DNA of early mouse embryos. Jaenisch injected retrovirus into early mouse embryos and showed that leukemia DNA sequences had integrated the mouse genome and also to its offspring. Jaenisch’s mice were the first transgenic animals in history.

Jaenisch is a leader in the field of therapeutic cloning, also known as nuclear transfer, in which the genetic information from one cell is transplanted into an unfertilized egg that has had its DNA removed. When it is placed in a Petri dish, the egg develops into a blastocyst from which stem cells can be harvested. Jaenisch’s therapeutic cloning research deals exclusively with mice, but he is an advocate for using the same techniques with human cells in order to advance embryonic stem cell research. However, Jaenisch opposes human reproductive cloning, where the egg is placed into the uterus of a female, with the hope that it will develop into a fetus.