DNA first became criminal evidence in 1986, when British police used a new genetic profiling technique to solve a double murder in Leicestershire, England. Within two years, the technology crossed the Atlantic and secured its first American conviction. In the four decades since, DNA has gone from an experimental lab technique to the single most powerful tool in forensic science, reshaping how crimes are solved and how wrongful convictions are overturned.
The 1984 Breakthrough That Started It All
In 1984, geneticist Alec Jeffreys at the University of Leicester discovered that certain regions of human DNA vary so dramatically between individuals that they can serve as a biological fingerprint. His technique, called DNA fingerprinting, used enzymes to cut DNA at specific points and produce a pattern of fragments unique to each person. It was originally developed for paternity testing and immigration disputes, not crime scenes.
That changed in 1986. Two teenage girls had been raped and murdered in the nearby villages of Narborough and Enderby in 1983 and 1986. Police had a suspect in custody who had confessed to one of the killings. When they asked Jeffreys to confirm the match, his DNA tests did the opposite: they proved the suspect was innocent, making this the first time DNA evidence exonerated someone in a criminal investigation.
Police then launched what became the world’s first genetic dragnet, collecting blood samples from over 4,000 local men. The real killer, Colin Pitchfork, tried to evade the dragnet by persuading a coworker to submit a sample in his place. When that coworker mentioned the scheme at a pub, someone reported it. Pitchfork’s DNA matched the crime scene evidence, and he was convicted in 1987. It was the first criminal conviction secured through DNA profiling.
DNA Evidence Enters American Courts
The United States followed almost immediately. In 1987, a Florida man named Tommie Lee Andrews was convicted of rape based on DNA evidence, becoming the first American defendant found guilty using the technology. He was sentenced to 22 years in prison. Courts across the country took notice, and within a few years, prosecutors and defense attorneys alike were introducing DNA results in trials ranging from sexual assault to homicide.
But admitting DNA into court required clearing a legal hurdle: judges needed a framework for deciding whether this new science was trustworthy enough to show a jury. Before 1993, most courts relied on a 1923 standard requiring that scientific evidence be “generally accepted” in its field. That changed with the Supreme Court’s 1993 decision in Daubert v. Merrell Dow Pharmaceuticals, which gave trial judges broader authority to evaluate scientific evidence. Under the Daubert standard, judges consider whether a technique can be tested, whether it has known error rates, whether it has undergone peer review, and whether it is accepted by the relevant scientific community. DNA profiling met all four criteria comfortably, and the Daubert framework became the dominant test for admitting forensic evidence in federal courts.
How the Technology Evolved
Early DNA profiling used a method called restriction fragment length polymorphism, or RFLP, which required relatively large, well-preserved samples of blood or tissue. That limited its usefulness at many crime scenes where only trace amounts of biological material were available.
In the early 1990s, a technique called PCR (polymerase chain reaction) changed the game. PCR essentially photocopies tiny segments of DNA millions of times, making it possible to generate a profile from far smaller or more degraded samples. The first use of PCR in a criminal trial actually dates to 1986, in a Pennsylvania case, but the technology didn’t become the forensic standard until the mid-1990s. By then, labs had adopted short tandem repeat (STR) analysis, which examines specific repeating sequences at multiple points across the genome. STR profiling quickly replaced the older methods and remains the backbone of forensic DNA work today.
The practical impact has been enormous. In 1995, labs needed about 2 nanograms of DNA to produce a usable profile. Current testing kits can work with as little as 0.4 nanograms, a fivefold reduction. That means a smudge of skin cells on a doorknob or a trace of saliva on a cigarette butt can now yield enough genetic material to identify a suspect.
National DNA Databases
In 1998, the FBI launched its national DNA database, known as CODIS (Combined DNA Index System). The system allows local, state, and federal labs to upload DNA profiles from crime scenes and convicted offenders, then search for matches across jurisdictions. By April 2021, CODIS had surpassed 20 million profiles, making it one of the largest forensic databases in the world.
A standard CODIS profile examines 13 or more locations on the genome. The probability that two unrelated people share the same full profile is astronomically small, on the order of 1 in a quadrillion. Even accounting for the size of the U.S. population and the millions of profiles in the database, researchers estimate the chance that a database search produces an erroneous match is roughly 1 in 3.4 million.
DNA as a Tool for Exoneration
DNA didn’t just help convict the guilty. It became the most powerful mechanism for freeing the wrongly convicted. Between 2000 and 2010, DNA was the primary evidence used to overturn wrongful convictions in 40% of 572 exoneration cases. Before modern DNA testing became available, people who were wrongfully convicted spent an average of nearly nine years in prison before being cleared. After 1989, when post-conviction DNA testing became more accessible, that average dropped to about 5.9 years.
Organizations like the Innocence Project have used DNA testing to challenge convictions built on less reliable evidence, such as eyewitness identifications or outdated forensic methods like bite-mark analysis. In many of these cases, biological evidence collected at the crime scene had been preserved for years or decades, and new testing revealed that the convicted person’s DNA simply wasn’t there.
Genetic Genealogy and the Golden State Killer
The most dramatic recent expansion of DNA as evidence came in 2018, when investigators identified Joseph James DeAngelo as the suspected Golden State Killer. He had evaded capture for over 40 years. The breakthrough didn’t come from CODIS. Instead, investigators uploaded crime scene DNA to GEDmatch, a publicly accessible genealogy database where people voluntarily share their genetic data to find relatives. The crime scene DNA matched to a probable fourth cousin of the suspect. Working with a genealogist, investigators built out the family tree until they narrowed the pool to DeAngelo.
This technique, called investigative genetic genealogy, doesn’t produce a direct match to a suspect. It identifies distant relatives, and detectives then use traditional investigative work to zero in on a specific individual. At that point, they collect a fresh DNA sample from the suspect (often from discarded items like a coffee cup, which courts have generally ruled carries no expectation of privacy) and compare it directly to the crime scene evidence. That direct comparison is the actual forensic evidence presented at trial.
Rapid DNA and What’s Happening Now
The latest shift is speed. Rapid DNA technology can produce a full genetic profile from a cheek swab in one to two hours, without a laboratory or human analyst. Since the Rapid DNA Act of 2017, the FBI has been rolling out these systems at law enforcement booking stations. When someone is arrested for a qualifying offense, their DNA can be processed on-site and searched against CODIS and a specialized database of unsolved violent crimes, all while the person is still in custody.
As of July 2025, national standards also allow Rapid DNA instruments to process forensic evidence from crime scenes, provided the work falls under an accredited lab’s oversight and the results are reviewed by qualified analysts before being entered into CODIS. This marks a significant expansion from the technology’s initial use, which was limited to known reference samples from arrestees.
From a single discovery in a British university lab in 1984 to automated machines generating profiles during police bookings, DNA evidence has reshaped criminal justice in roughly 40 years. Its power lies not just in identifying the guilty, but in proving innocence, a dual role no previous forensic tool had ever reliably filled.