Identical twins are put on trial. Can DNA tests tell them apart?

In a French criminal trial, conventional DNA analysis failed to distinguish twin brothers, but emerging scientific methods could prove useful in such cases.

By Rachel Fieldhouse & Nature magazine

An identical twin commits a crime and leaves his DNA at the scene. Can DNA testing reveal which twin is the culprit?

The question reportedly arose in a case in France last month in which the DNA of twin brothers was found on a gun, but conventional DNA testing failed to determine which brother the gun belonged to. The men are monozygotic twins – the result of a single egg splitting into two after being fertilized by a single sperm – meaning they have the same DNA.

When DNA is found at a crime scene, forensic scientists typically attempt to identify the person it belongs to using a technique called short tandem repeat (STR) analysis. The technique uses the polymerase chain reaction (PCR) method to amplify up to 30 specific regions of the genome that have many genetic variations, explains Brendan Chapman, a forensic pathologist and researcher at Murdoch University in Perth, Australia.

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These amplified regions are sequenced to determine the number of STRs and their base pair patterns, which can then be compared to pedigree databases or STR profiles of suspects to identify potential matches. Ordinarily, says Chapman, “we can tell one person from another very easily.” But with monozygotic twins, “there is simply no difference” in these regions of the genome.

Deep dive

Whole genome sequencing can help distinguish identical twins. By analyzing a person’s entire genome, scientists can identify differences caused by mutations that occur after an egg divides. These changes are rare, says Chapman. For example, a 2014 study identified only five genetic changes in a pair of adult twins.

There have been rare cases in which whole genome sequencing has helped courts distinguish between twins, says Xanthé Weston, a criminologist and researcher at Central Queensland University in Mackay, Australia. But whole genome sequencing requires obtaining a sufficient amount of DNA to analyze, Weston adds.

Other researchers have reported success in telling twins apart by sequencing the DNA found in mitochondria, the powerhouses of cells. Compared to nuclear DNA, used in genome sequencing and STR analysis, mitochondrial DNA (mtDNA) mutates more often, meaning it is more likely to differ between twins.

U.S. courts have admitted mtDNA analysis as evidence since the mid-1990s. But it hasn’t been used in cases involving twins, Weston says.

“Epigenetic” changes

Another promising technique involves adding methyl groups to DNA, a process called methylation, which can change how genes work. These “epigenetic” changes can be caused by factors such as a person’s behaviors – such as their diet and drinking or smoking habits – and their environment.

Last year, a team of South Korean scientists sequenced the genomes of 54 pairs of newborn monozygotic twins. After identifying 711 sites where methylation generally differed, they selected and tested various combinations, including five that were particularly variable. By comparing the extent of methylation at these five sites, the team was able to differentiate 50 of the 54 pairs of twins. When they repeated the experiment on two cohorts of adults, they managed to differentiate 41 of the 47 pairs and 105 of the 118 pairs of twins.

DNA methylation analysis has not yet been used to distinguish twins in court. And Chapman notes that this type of analysis might not be able to distinguish twins who have similar variations caused by a similar lifestyle or exposure to similar environmental factors such as pollution.

Challenges remain

The new techniques are promising, Chapman adds, but there are still challenges to overcome before they can be used in forensic investigations. For example, they all require relatively large amounts of DNA, but samples taken from crime scenes often contain only small amounts or are in poor condition.

Analyzes can also be expensive and time-consuming, in part because of the work required to demonstrate to a courtroom that the results are conclusive enough to be considered evidence. Last year, whole genome sequencing was admitted for the first time in a US murder trial.

Weston says it’s important to combine DNA evidence with other types of evidence, such as fingerprints, when prosecuting a person because there are many reasons why a person’s DNA may be present at a crime scene. “I would never want to see someone prosecuted based solely on their DNA,” she adds.

This article is reproduced with permission and has been published for the first time March 5, 2026.