Maxine Frank Singer was a researcher who studied molecular biology and genetics in the United States during the twentieth and twenty-first centuries. Her work synthesizing RNA molecules advanced researchers’ ability to understand triplets of nucleotides in RNA and DNA, which allowed them to read the genetic code. Singer was also one of the first scientists to find that certain long repeated DNA sequences, called long interspersed nucleotide elements, or LINEs, can jump around, and the mechanism behind it. Outside of her research, Singer also was active in science policy, helping to regulate the use of genetic engineering and recombinant DNA technologies, and organizing conferences around the topic, such as the Asilomar Conference. Prior to Singer’s work, researchers knew that DNA was a double stranded molecule made up of alternating nucleotides, but Singer contributed to researchers’ understanding of what those nucleotides meant in the genetic code. While Singer advanced the scientific community’s understanding of how to read the genetic code and how LINEs impact genetic diseases, her promotion of ethical discussions of scientific responsibilities in manipulating the code helped create policy that continues to affect researchers exploring genetic engineering as of 2024.

Heinrich Hermann Robert Koch was a researcher from Germany who studied the causative agents of infectious diseases in various parts of Europe, Africa, and Asia during the late nineteenth and early twentieth centuries. Koch developed what researchers call Koch’s postulates, which are four criteria designed to establish whether a bacterium causes a certain disease, and as of 2025, many researchers still use Koch’s postulates to guide their research. He also received the Nobel Prize in Physiology or Medicine for his discovery of Mycobacterium tuberculosis, which is the bacterium responsible for tuberculosis, an infectious disease that primarily attacks the lungs. Koch’s research on identifying disease-causing bacteria for various infectious diseases has advanced disease prevention and treatment, especially for tuberculosis, which has the ability to transmit from mother to child.

Lewis Madison Terman was a researcher and university professor who studied educational psychology and advocated for eugenics in the United States during the early twentieth century. The US eugenics movement, which Terman supported, was a collection of scientific research and social programs that aimed to improve human populations through control over human reproduction. One area many eugenicists studied was human intelligence as a means of determining how “desirable” a person may be. During the 1910s, while working at Stanford University in Palo Alto, California, Terman helped devise the Stanford-Binet scale for intelligence testing. As of 2024, the Stanford-Binet test is one of the main methods for providing individual intelligence quotient, or IQ, scores. In addition to the Stanford-Binet scale, Terman promoted the idea that individuals had fixed and inherited capacities for intelligence. Through both his development of a widely used method for measuring human intelligence and his promotion of the idea of intelligence as hereditary, Terman supported widespread social efforts to control human reproduction in the US during the twentieth century.

Albert Ludwig Sigesmund Neisser was a physician and scientist working in Poland who, in 1879, identified the bacterium that causes gonorrhea. Before Neisser’s discovery, physicians and scientists were unsure of what causes gonorrhea, a sexually transmitted infection, or STI, that typically causes genital pain and discharge in those infected. Using newly available microscopy techniques, Neisser examined genital discharge from patients with gonorrhea and observed the bacteria that eventually became known as Neisseria gonorrhoeae, named in his honor. Beyond discovering the bacterial cause of gonorrhea, Neisser also directed a dermatology clinic in Breslau, Poland, and researched other diseases including syphilis and leprosy. In addition to his discoveries, he also experienced scandals and ethical controversies regarding his research practices, which resulted in the implementation of directives regarding informed consent. By identifying the causative agent of gonorrhea, Neisser enabled future physicians and scientists to develop treatments for a disease that can cause infertility and be passed from mother to child causing serious illness in infants.

Susumu Ohno studied chromosome structure and evolution during the twentieth century in the United States. Ohno was one of the first researchers to propose that the Barr body, a mass of genetic material within female mammalian cells, was a condensed X-chromosome. Ohno also developed a theory that gene duplication, when specific regions of a chromosome become multiplied, is a primary driver of evolution, with natural selection playing a secondary role. He theorized that gene duplication allows one copy of the original gene to remain and perform its normal function while a second copy of the gene can mutate and undergo natural selection, leading to diversification of life. Later in his career, Ohno composed music based on DNA sequences. As of 2025, researchers continue to debate Ohno’s theory of gene duplication. Through his research, Ohno introduced a new perspective on the driving forces of evolution, which advanced researchers’ understanding of chromosomal evolution and genetic diversity.

James Daniel Hardy was a surgeon and researcher practicing in the United States during the twentieth century who studied organ transplantation, or the transfer of an organ from a donor to another individual. In 1963, he performed one of the first recorded lung transplants from a human lung donor. The transplant was successful for three weeks before the patient died of kidney failure. In 1964, Hardy also performed one of the first human heart transplants with a chimpanzee donor, and the transplanted heart pulsed for ninety minutes in the patient’s chest prior to death. He also collaborated on one of the first successful uterus and ovary transplants in a dog, in 1966. Hardy’s research on organ transplantation helped paved the way for improved forms of the technique, which as of 2025 saves the lives of millions of people every year.

Ernest Armstrong McCulloch was a medical researcher who studied the biology of blood cells and blood cancer in Toronto, Ontario, in the second half of the twentieth century, and who, with James Till, demonstrated the existence of stem cells in the blood, called hematopoietic stem cells. Stem cells are cells that can develop into more specialized cells in the body. Hematopoietic stem cells, or HSCs, are a type of stem cell that, when present in blood and bone marrow, can develop into specialized blood cells, such as red blood cells, white blood cells, and platelets. Since the start of the twentieth century, researchers hypothesized the existence of something with the self-renewal properties of what were later called stem cells but lacked evidence identifying those stem cells. McCulloch’s work provided the necessary evidence, which laid the foundation for identifying the function of stem cells in other tissues. Through his work leading to the identification of blood stem cells, McCulloch aided the development of treatments for blood cancers, which affect roughly 1.6 million people in the United States.

Irving Lerner Weissman is a researcher and professor in developmental biology at the Stanford University School of Medicine in Stanford, California. Weissman is also a professor of pathology and Virginia & D. K. Ludwig professor of clinical investigation in cancer at the Stanford University School of Medicine. Weissman studies the biology of stem cells and immune cells and has conducted research in those fields during the late twentieth and early twenty-first centuries. In the late 1980s, Weissman’s team developed methods to identify hematopoietic stem cells, or HSCs, which give rise to the body’s blood and immune cells. Also, in the early 2000s, Weissman also co-authored California’s Proposition 71, which secured three billion dollars in state funding for stem cell research after the federal government restricted human embryonic stem cell work. Weissman’s efforts have demonstrated the therapeutic potential of stem cell transplantation in the treatment of diseases, including blood cancer.

Oliver Smithies researched physical chemistry, biochemistry, and genetics in England, Canada, and the United States during the twentieth and early twenty-first centuries and contributed to the study of gene function. During the 1950s, Smithies developed a technique to improve separating proteins based on their physical properties. Later, in the 1980s, Smithies utilized homologous recombination, a process that involves two similar pieces of deoxyribonucleic acid, or DNA, that exchange information, to target and manipulate specific genes. Smithies’s research on homologous recombination helped lead to the creation of the knockout mouse, a model organism that has genetic alterations to a single gene, to help researchers understand the function of genes in development. In 2007, Smithies received the Nobel Prize in Physiology or Medicine along with Sir Martin Evans and Mario Capecchi for work on introducing specific gene modifications in mice. Smithies’s scientific contributions toward developing the knockout mouse provided a basis for subsequent research studying the impact of different genes on human health.

Charles Richard Drew was an African American surgeon who helped improve blood transfusion practices during World War II and contributed to the development of modern-day blood banking. Sometimes called the Father of the Blood Bank, Drew showed that blood plasma, or the liquid component of blood, could be safely separated from whole blood, stored, and used for transfusion. Plasma has several advantages over whole blood, including that it can be stored safely for longer and transported over long distances. Drew’s methods allowed medics to treat thousands of injured Allied soldiers who were suffering from blood loss. Drew was also an advocate for racial desegregation in the United States and fought against the discriminatory blood donation practices of the American Red Cross. Drew’s work made blood transfusions safer and more accessible, which not only helped the Allied war effort in World War II, but also led to improvements in the treatment of complications during childbirth and efforts to reduce maternal mortality rates, particularly among Black women.