SINCE its first use in the 1980s -- a breakthrough dramatised in recent ITV series Code of a Killer -- DNA profiling has been a vital tool for forensic investigators. Now researchers at the University of Huddersfield have solved one of its few limitations by successfully testing a technique for distinguishing between the DNA -- or genetic fingerprint -- of identical twins. 
 
 The probability of a DNA match between two unrelated individuals is about one in a billion. For two full siblings, the probability drops to one-in-10,000. But identical twins present exactly the same DNA profile as each other and this has created legal conundrums when it was not possible to tell which of the pair was guilty or innocent of a crime. This has led to prosecutions being dropped, rather than run the risk of convicting the wrong twin. 
 
 Now Dr Graham Williams and his Forensic Genetics Research Group at the University of Huddersfield have developed a solution to the problem and published their findings in the journal Analytical Biochemistry. 
 
 Previous methods have been proposed for distinguishing the DNA of twins. One is termed "mutation analysis," where the whole genome of both twins is sequenced to identify mutations that might have occurred to one of them. 
 
 "If such a mutation is identified at a particular location in the twin, then that same particular mutation can be specifically searched for in the crime scene sample. However, this is very expensive and time-consuming and is unlikely to be paid for by cash-strapped police forces," according to Dr Williams, who has shown that a cheaper, quicker technique is available. 
 
 It is based on the concept of DNA methylation, which is effectively the molecular mechanism that turns various genes on and off. 
 
 As twins get older, the degree of difference between them grows as they are subjected to increasingly different environments. For example, one might take up smoking, or one might have a job outdoors and the other a desk job. This will cause changes in the methylation status of the DNA. 
 
 In order to carry our speedy, inexpensive analysis of this, Dr Williams and his team propose a technique named "high resolution melt curve analysis" (HRMA). 
 
 "What HRMA does is to subject the DNA to increasingly high temperatures until the hydrogen bonds break, known as the melting temperature. The more hydrogen bonds that are present in the DNA, the higher the temperature required to melt them," explains Dr Williams. 
 
 "Consequently, if one DNA sequence is more methylated than the other, then the melting temperatures of the two samples will differ -- a difference that can be measured, and which will establish the difference between two identical twins." 
 
 HRMA has some limitations, acknowledges Dr Williams. For example young twins, or twins raised in highly similar environments may not have yet developed sufficient methylation differences. 
 
 Also the technique requires a high sample quantity that might not be present at the crime scene. 
 
 "Nevertheless, we have demonstrated substantial progress towards a relatively cheap and quick test for differentiating between identical twins in forensic case work," says Dr Williams, who gives a detailed summary of the science behind the breakthrough at blog-site The Conversation. ||||| Photo Advertisement Continue reading the main story 
 
 BOSTON — The rape suspect, court papers said, was identified by a longtime friend who said they committed the crime together. The suspect’s DNA was tied to the crime. But prosecutors hoping to present an airtight case had a concern: The suspect, Dwayne McNair, has an identical twin, meaning the DNA testing pointed not to one person, but two. 
 
 
 
 
 
 
 
 
 
 It was a twist worthy of a crime novel, but one that has cropped up from time to time in cases both in the United States and Europe, including an earlier one in Boston. Because monozygotic twins, the scientific name for identical twins, come from a single fertilized egg — the same genetic material — they cannot be distinguished by conventional DNA testing, confounding prosecutors who might otherwise be able to build a solid case. 
 
 “There certainly have been instances where identical twins have laid blame on the other, or each denied responsibility, and for a lack of persuasive evidence they’ve both gotten off,” said Jennifer Mnookin, a professor of law at U.C.L.A. who said such cases were rare. 
 
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 Prosecutors here are hoping that Mr. McNair’s case will be a breakthrough for criminal cases involving identical twins. On Monday, Mr. McNair pleaded not guilty to eight counts of aggravated rape and two counts of armed robbery in two separate attacks he is accused of committing in 2004. 
 
 To make their case in court, prosecutors want to present evidence from a cutting-edge DNA test, performed by a European company, that they say shows that Mr. McNair, and not his twin brother, Dwight, committed the attacks. 
 
 “We’ll be the first jurisdiction in the U.S. to bring this evidence forward, if it is acceptable,” said the Suffolk County district attorney, Daniel F. Conley, after Mr. McNair was taken into custody on Monday. 
 
 According to court papers, it was 2007 when the police first suspected Mr. McNair of committing two abductions and rapes that occurred in September 2004. His DNA matched that of semen recovered from the second victim — but a DNA sample taken from his brother Dwight matched, too, and investigators lacked evidence to give them probable cause to arrest either brother, the papers said. 
 
 In 2011, the court papers said, the police identified another suspect, Anwar Thomas, who said he had carried out the attacks with Dwayne McNair. Mr. Thomas pleaded guilty, with a cooperation agreement, in 2012, and Mr. McNair was indicted that fall, the papers said. 
 
 Mr. McNair was set to be tried in April of this year, but then David Deakin, an assistant district attorney working on the case, learned about the new testing, by a company called Eurofins. Mr. Conley’s office decided to pursue a legal procedure allowing them to terminate the case and start over later, with a new indictment, once the testing was complete. 
 
 “There is confirmation that the assailant in both cases is the defendant, Dwayne McNair,” said Mr. Conley, who said the testing may well prove useful to other prosecutors dealing with similar cases. 
 
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 A judge will determine whether or not to admit the test results as evidence at a hearing before the trial. Ms. Mnookin, the law professor, said the introduction of any new testing method required careful consideration. 
 
 “I don’t know of any earlier case in the U.S. where they’ve tried to do a test like this,” Ms. Mnookin said. She added, “They should make sure the tests are ready for prime time.” 
 
 Robert Tobin, the lawyer for Mr. McNair, said he expected that he would argue the testing is not a reliable process. During Mr. McNair’s arraignment, he said the victim’s physical description of their assailant “much more nearly fits that of his twin brother.” 
 
 Mr. Conley and Mr. Deakin in 2006 convicted an accused rapist with a twin brother, after the case twice ended in mistrials. In 2004, a juror in one of the trials told The Boston Globe the jury had deadlocked when nine jurors supported conviction, while three argued that the DNA evidence could have come from his twin. 
 
 “The fact that the DNA matches both identical twins created some doubt in all of our minds, but that doubt weighed more for some and much less for others,” said the juror, Matthew Dullea.
– Identical twins share an identical DNA profile, and when you're an investigator examining DNA evidence, that can be a problem. The cases may be rare, but they exist, legal expert Jennifer Mnookin told the New York Times last year amid a rape case involving a suspect with a twin: "There certainly have been instances where identical twins have laid blame on the other, or each denied responsibility, and for a lack of persuasive evidence they’ve both gotten off." But new research could help prosecutors get past that hurdle, Science Daily reports. British scientists have found that by heating DNA to its "melting temperature," they can distinguish between twins. In what the researchers call "high resolution melt curve analysis," heat is applied to DNA; the melting temperature refers to the temperature at which the hydrogen bonds in our DNA break. Environmental factors can alter the number of hydrogen bonds in a DNA sequence: One twin could be a smoker, for instance, or be exposed to more sunlight. More bonds lead to a higher melting temperature. To arrive at their findings, published in Analytical Biochemistry, researchers had five sets of identical twins supply their DNA via a cheek swab; using HRMA and looking at two genetic markers, the researchers were able to distinguish between all five sets in one case, and four of the five in the case of the second marker. But some issues remain, lead researcher Dr. Graham Williams notes. One is that the process requires "a high sample quantity" of DNA that might not be available at a crime scene. (This week, a man's charges were dropped—in part thanks to DNA evidence—after he had spent 36 years in prison.)