David Wildt's cheetah (Acinonyx jubatus) research from 1978-1983 became the foundation for the use of embryological techniques in endangered species breeding programs. The cheetah is a member of the cat family (Felidae), which includes thirty-seven species. According to the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) all Felidae species are currently threatened or endangered, with the exception of the domestic cat (Felinus catus). Cheetahs are an internationally recognized charismatic megafauna species, prized zoo specimens, difficult to breed, and the basis of many conservation campaigns. Like most species, cheetahs have not traditionally been studied; only a few "model" organisms have been thoroughly researched in a laboratory setting. This research revealed that the difficulty observed in breeding cheetahs in captivity is due to their lack of genetic diversity.

Conrad Hal Waddington's "Experiments on Embryonic Induction III," published in 1934 in the Journal of Experimental Biology, describes the discovery that the primitive streak induces the mammalian embryo. Waddington's hypothesis was that a transplanted primitive streak could induce neural tissue in the ectoderm of the rabbit embryo. The primitive streak defines the axis of an embryo and is capable of inducing the differentiation of various tissues in a developing embryo during gastrulation. In this experiment Waddington was, in fact, able to induce neural differentiation. Waddington noted that the tissue is "competent"; for a chick organizer, and by deduction a mammalian organizer must exist. Competence refers to a cell's ability to respond to an inducing signal, which is temporally limited to certain developmental stages. Waddington's initial work laid the foundation for many decades of research to follow, including further experiments by Waddington with the mammalian organizer.

In 1932, the United States Public Health Service, or USPHS, began the Tuskegee Syphilis Study, initially known as the Tuskegee Study of Untreated Syphilis in the Negro Male, as an experiment to understand the effects of untreated syphilis in Black men. When the study began, there was no known cure for syphilis. The study involved 600 Black men in Macon County, Alabama, and took place on the campus of Tuskegee Institute, now Tuskegee University. The study leaders did not tell the men the truth about the purposes, risks, or benefits of the study. Moreover, despite the development of an effective drug against syphilis—penicillin, available beginning in the late 1940s—the study investigators withheld treatment and continued to examine the untreated progression of the disease in the men. The study went on for forty years. The US government officially shut it down in 1972 after information about the study leaked to the public. The researchers’ unethical practices, including not obtaining informed consent from the participants, led to changes in federal laws regarding human clinical trials and to the creation of the Belmont Report, which outlines ethical human research guidelines.

In the 1990s, John E. Dick and Dominique Bonnet, researchers at the University of Toronto, in Toronto, Ontario, investigated how a blood cancer called acute myeloid leukemia, or AML, arises from blood-forming cells. Researchers in the field at the time did not know which cells initiate and maintain AML in the body, and there were conflicting hypotheses about which cells were responsible. Scientists hypothesized that AML might be mainly composed of more specialized and differentiated blood cells. Dick and Bonnet conducted experiments in which they grew human AML cells in mice and investigated which cells initiated and maintained the cancer to explore that hypothesis. Their study was one of the first to use sensitive mouse models to characterize and uncover which cells initiated and comprised AML, which contributed to the understanding of its cellular origin and aided in the development of targeted therapies for leukemia treatment.