A lymphoblastoid cell line, or LCL, is an immortalized population of cells derived from a specific type of white blood cell called a B lymphocyte that scientists around the world began using for biomedical research in the late 1960s. By immortalized, scientists mean that the cells have been altered so they can grow and divide indefinitely, or at least for an extended period of time. That trait of LCLs makes them useful as a replenishable source of cells and the DNA contained within them. Scientists obtain LCLs by first collecting a blood sample and then exposing the B lymphocytes in the blood to Epstein-Barr virus, or EBV. EBV alters the B lymphocytes in such a way that the cells begin to multiply without restraint. Researchers began making and storing LCLs from individuals around the world in the 1960s. As of 2025, LCLs form a mainstay of biomedical research, especially in human genetics and genomics.

Lewis Albert Sayre was an orthopedic surgeon who practiced medicine in New York City, New York, during the second half of the nineteenth century and held a number of leadership positions in his profession. Over the course of his nearly fifty-year career, Sayre developed a number of surgical and nonsurgical treatments of bone problems including scoliosis and other forms of spinal curvature, club foot, and hip-joint disease resulting from tuberculosis infection. He also helped popularize circumcision, or surgical removal of the foreskin of the penis, as a treatment for a variety of medical conditions, including muscle paralysis and epilepsy, based on the theory, discredited as of 2025, that a chronically irritated foreskin could lead to diseases in other parts of the body. By providing a medical justification for circumcision, Sayre helped to embed the surgical procedure within US medicine, even as the medical rationales for its use would change periodically in the decades that followed.

In 1949, Douglas Gairdner, a pediatrician in Cambridge, England, published “Fate of the Foreskin: A Study of Circumcision,” hereafter, “Fate of the Foreskin,” in the British Medical Journal. In the article, Gairdner highlights what he saw as a seriously understudied topic, the natural development of the foreskin in males. Although physicians were then circumcising tens of thousands of male infants annually in England, data on the normal anatomy and function of the foreskin were scarce. In “Fate of the Foreskin,” Gairdner assembles those data and uses them to argue against performing circumcision to treat conditions like phimosis. Phimosis is when the foreskin tightly encases the glans, or head, of the penis and cannot retract. Gairdner finds that an unretractable foreskin is actually the normal state for newborn males, and that the foreskin will become retractable on its own over a period of months to years. By showing that phimosis is not a pathological condition, “Fate of the Foreskin” questioned the legitimacy of routine circumcision, and ultimately led to a steep decline of the practice in England.

The Human Genome Diversity Project, or HGDP, was an effort led by US-based scientists to collect DNA from members of Indigenous communities living around the world for the purpose of understanding human history, migration, and evolution. Launched in 1991, and led by Luca Cavalli-Sforza, a scientist at Stanford University in Stanford, California, the HGDP initially had the support of US funding agencies. However, the project eventually lost that support when representatives of Indigenous groups protested the project as being exploitative and fellow scientists accused it of racism. Though the project ultimately failed to collect most of the samples it had originally planned, the HGDP was one of the first attempts by scientists to catalogue worldwide human genetic variation, and the DNA samples it did collect formed the basis of many subsequent research studies concerned with understanding human genetic variation and migration patterns.

The Gomco circumcision clamp is a metal device that medical practitioners use to perform circumcision, or the removal of the foreskin of the penis. Created in 1934 by Hiram S. Yellen, a physician who studied obstetrics and gynecology, and inventor Aaron A. Goldstein, the Gomco clamp was one of the first and, as of 2025, is the most commonly used circumcision clamp in the United States. To use the Gomco clamp, the medical practitioner first separates the foreskin from the glans, or head, of the penis, then places the foreskin into the clamp. The practitioner tightens the clamp, which crushes a thin, circular ribbon of foreskin, and then cuts off the remaining foreskin above the crushed portion. The Gomco clamp reduces the risk of blood loss, controls the amount of foreskin removed, protects the glans of the penis during circumcision, and allows for a clean surgical cut. By simplifying the surgical procedure of circumcision and reducing the risk of complications, the Gomco clamp helped to institutionalize routine, non-therapeutic infantile circumcision as part of the childbirth process in US hospitals.

Non-therapeutic infant circumcision is the surgical removal of healthy foreskin from a male infant, often shortly after birth, for the purpose of achieving potential future medical benefits. Today, in 2025, the practice is common the United States but not as common in other Western industrialized countries. Though circumcision itself is an ancient cultural practice, doctors began performing circumcision for medical purposes only in the nineteenth century, and primarily in English-speaking countries. Orthopedic surgeon Lewis Sayre, who practiced medicine in New York City, New York, in the late nineteenth century popularized circumcision as a treatment for conditions such as muscle paralysis. Sayre’s ideas eventually fell out of favor, but doctors increasingly identified other reasons to perform the procedure, including the prevention of sexually transmitted diseases, urinary tract infections, and cancer. As of 2025, doctors, parents, ethicists, and others continue to debate the medical value of circumcision as well as the ethics of operating on the healthy genitals of people who cannot consent.

Launched in 2002, the International HapMap Project was a collaborative effort among scientists from around the world to create a map of common patterns of genetic variation in the human genome. HapMap stands for haplotype map. A haplotype is a stretch of DNA nucleotides, or letters, that individuals inherit as a block because they lie relatively close together along a chromosome. For any particular region of a chromosome, there may be multiple different haplotypes present among humans, each characterized by a slightly different DNA sequence. By collecting and sequencing the DNA of initially 270 individuals from several different geographic regions, HapMap scientists were able to identify common haplotypes that exist among those individuals, as well as reliable markers to distinguish them. That collection of haplotypes and identifying markers—the HapMap—provided a shortcut for researchers who wanted to identify associations between those inherited DNA variants and particular human traits, especially common, complex diseases like heart disease and cancer.

The 1,000 Genomes Project, which began in 2008, was an international effort to create a detailed and publicly accessible catalog of human genetic variation to support medical studies aimed at exploring genetic contributions to disease. Project scientists sequenced the entire genomes of 2,504 individuals from around the world—more than the 1,000 originally planned. The Project extended the results of the International HapMap Project, a prior effort at cataloging human genetic variation that ran from 2002 through 2010. Whereas the HapMap identified common genetic variants, meaning specific DNA sequences present in five percent or more of individuals in a population, the 1,000 Genomes Project identified genetic variants present in as few as one percent of individuals in a population. By assembling a larger catalog of DNA sequence variation than had previously existed, the 1,000 Genomes Project paved the way for researchers to more precisely locate disease-related genetic variation passed from parent to child.

A genome-wide association study, or GWAS, is a method for identifying variations in DNA that may contribute to the development of a particular trait, such as a disease. A GWAS relies on identifying statistical correlations between many, often thousands of, DNA markers and a particular trait. Scientists employ GWASs to try to identify the genetic contributions to complex traits, such as common human diseases. Complex traits are ones that scientists suspect are the result of multiple genes and environmental inputs acting together, in contrast to simple, Mendelian disorders that result primarily from the disturbance of a single gene. The genetic variants identified through a GWAS typically account for only a small proportion of the expected genetic contribution to a complex trait, which scientists refer to as the missing heritability problem. Since 2006, scientists have conducted thousands of GWASs aimed at identifying the genetic contributions to complex traits and have identified many thousands of genetic variations that correlate with those traits, although as of 2025, because of the missing heritability problem and other limitations, the concrete contributions of GWASs to medicine have so far been modest.