In 2005, Ernest McCulloch and James Till published the article “Perspectives on the Properties of Stem Cells,” which discusses the various properties and future possibilities for the use of stem cells. Stem cells are unspecialized cells that can develop into several different cell types. In the article published in the journal Nature on 1 October 2005, the authors say they wrote the article to dispel misconceptions about what stem cells are, what they do, address some controversies surrounding stem cells, and discuss potential uses of stem cells. In the article, McCulloch and Till reveal how stem cell research has revolutionized cancer treatment as well as set the stage for future embryonic and adult stem cell research.

Walter Jakob Gehring discovered the homeobox, a DNA segment found in a specific cluster of genes that determine the body plan of animals, plants, and fungi. Gehring identified the homeobox in 1983, with the help of colleagues while isolating the Antennapedia (Antp) gene in fruit flies (Drosophila) at the University of Basel in Basel, Switzerland. Hox genes, a family of genes that have the homeobox, determine the head-to-tail (anterior-posterior) body axis of both vertebrates and invertebrates. Gehring also identified the homeobox-containing Pax-6 gene as the master control gene in eye development of Drosophila, the same gene that, when mutated or absent in humans, leads to aniridia, or lack of the iris, in humans. Gehring's work with the homeobox suggested to biologists that widely different species share a similar and evolutionarily conserved genetic pathway that controls the development of overall body plans, from fruit flies to humans.

Edwin Stephen Goodrich studied the structures of animals in England during the nineteenth and twentieth centuries. Goodrich studied how animals develop to identify their parts and to establish the evolutionary relationships between different species. Goodrich established that body structures can shift their positions relative to an organism's body during evolution, and he hypothesized that body structures can share ancestry (homology) between organisms of different species, even without identical body placement. Goodrich claimed that any given characteristic of an organism results from both genetic and external sources.

In the early 2000s, Manjong Han, Xiaodang Yang, Jennifer Farrington, and Ken Muneoka investigated how genes and proteins in fetal mice (Mus musculus) influenced those fetal mice to regenerate severed toes at Tulane University in New Orleans, Louisiana. The group used hind limbs from mice to show how the gene Msx1 (Homeobox 7) functions in regenerating amputated digits. The researchers showed that in the process of regenerating digit tips, Msx1 genes make products that regulate or influence other genes, such as the Bone Morphogenetic Protein 4 gene (BMP4 gene), to produce proteins, such as the BMP4 proteins. The researchers also showed that BMP4 proteins, which are produced from the BMP4 gene, function in tissues during the process of limb development. Furthermore, while Msx1 genes regulate other genes during the process of regeneration, they don't produce proteins otherwise needed to organize cells in the regeneration of digit tissues. The group published their results in 2003 as Digit Regeneration Is Regulated by Msx1 and BMP4 in Fetal Mice.

Diethylstilbestrol (DES) is an artificially created hormone first synthesized in the late 1930s. Doctors widely prescribed DES first to pregnant women to prevent miscarriages, and later as an emergency contraceptive pill and to treat breast cancer. However, in 1971, physicians showed a link between DES and vaginal cancer during puberty in the children of women who had taken DES while pregnant. Consequently, the US Food and Drug Administration (FDA) banned its use during pregnancy. In the late 2000s, several studies showed that the grandchildren of women who had consumed DES also suffered medical issues. By the early decades of the twenty-first century, roughly ten million people in the US had been exposed to DES, and three generations of individuals had suffered medical issues due to DES exposure. Researchers class DES as an endocrine disruptor, which affects the form and function of the hormone (endocrine) system.

The Y-chromosome is one of a pair of chromosomes that determine the genetic sex of individuals in mammals, some insects, and some plants. In the nineteenth and twentieth centuries, the development of new microscopic and molecular techniques, including DNA sequencing, enabled scientists to confirm the hypothesis that chromosomes determine the sex of developing organisms. In an adult organism, the genes on the Y-chromosome help produce the male gamete, the sperm cell. Beginning in the 1980s, many studies of human populations used the Y-chromosome gene sequences to trace paternal lineages. In mammals, the Y-chromosomes contain the master-switch gene for sex determination, called the sex-determining region Y, or the SRY gene in humans. In most normal cases, if a fertilized egg cell, called a zygote, has the SRY gene, the zygote develops into an embryos that has male sex traits. If the zygote lacks the SRY gene or if the SRY gene is defective, the zygote develops into an embryo that has female sex traits.

In 2012, a team of scientists across the US conducted an experiment to find the mechanism that allowed a group of flatworms, planarians, to regenerate any body part. The group included Danielle Wenemoser, Sylvain Lapan, Alex Wilkinson, George Bell, and Peter Reddien. They aimed to identify genes that are expressed by planarians in response to wounds that initiated a regenerative mechanism. The researchers determined several genes as important for tissue regeneration. The investigation helped scientists explain how regeneration is initiated and describe the overall regenerative mechanism of whole organisms.

Carl Richard Moore was a professor and researcher at the University of Chicago in Chicago, Illinois who studied sex hormones in animals from 1916 until his death in 1955. Moore focused on the role of hormones on sex differentiation in offspring, the optimal conditions for sperm production, and the effects of vasectomy or testicular implants on male sex hormone production. Moore's experiments to create hermaphrodites in the laboratory contributed to the theory of a feedback loop between the pituitary and fetal gonadal hormones to control sex differentiation. Moore showed that the scrotal sac controls the temperature for the testes, which is necessary for sperm production. He also helped distinguish the hormones testosterone, and androsterone from testicular extracts.

Frank Rattray Lillie's research on freemartins from 1914 to 1920 in the US led to the theory that hormones partly caused for sex differentiation in mammals. Although sometimes applied to sheep, goats, and pigs, the term freemartin most often refers to a sterile cow that has external female genitalia and internal male gonads and was born with a normal male twin. Lillie theorized that a freemartin is a genetic female whose process of sexual development from an undifferentiated zygote was suppressed or antagonized by her twin's release of male hormones via their shared blood circulation in utero. Despite publications of similar findings by physician Julius Tandler in Vienna, Austria, in 1910 and physician Karl Keller in Wiesensteig, Germany in 1916 prior to Lillie's research, Lillie often receives credit for the hormonal theory of sex differentiation in the freemartin. Lillie's study of freemartins, and the subsequent research by graduate students in Lillie's laboratory at the University of Chicago in Chicago, Illinois, prompted many embryologists to research sex differentiation and hermaphroditism in mammals.