Earlier this month, the Texas Forensic Science Commission raised concerns about the accuracy of the statistical interpretation of DNA evidence, and it is now checking whether convictions going back more than a decade are safe. 
 
 Despite how it is often portrayed, in the media and in courts, the forensic science of DNA is far from infallible. Particularly concerning is that police and prosecutors now frequently talk of 'touch DNA' — genetic profiles of suspects and offenders that have been generated in a laboratory from just a handful of skin cells left behind in a fingerprint. 
 
 Research done by me and others at the University of Indianapolis in Indiana has highlighted how unreliable this kind of evidence can be. We have found that it is relatively straightforward for an innocent person's DNA to be inadvertently transferred to surfaces that he or she has never come into contact with. This could place people at crime scenes that they had never visited or link them to weapons they had never handled. 
 
 Such transfer could also dilute the statistics generated from DNA evidence, and thereby render strong genetic evidence almost insignificant. (The statistics issue is reportedly the focus of the Texas investigation.) 
 
 We urgently need to review how DNA evidence is assessed, viewed and described. Everyone in the medico-legal community — forensic scientists and technicians, DNA analysts, potential jurors, judges and lawyers for both the prosecution and defence— must know and understand the potential for mistakes. 
 
 The term 'touch DNA' conveys to a courtroom that biological material found on an object is the result of direct contact. In fact, forensic scientists have no way of knowing whether the DNA was left behind through such primary, direct transfer. It could also have been deposited by secondary transfer, through an intermediary. (If I shake your hand then I could pass some of your skin cells onto something that I touch next.) 
 
 Contamination from secondary DNA transfer was raised as a possible problem in Nature in 1997 (R. A. H. van Oorschot and M. K. Jones Nature 387, 767; 1997). It is known to happen, but has largely been dismissed by legal experts as being rare outside the conditions of a laboratory. Experiments done in real-world conditions seemed to support this, and concluded that secondary DNA transfer would have little impact on interpretation of the genetic profile. 
 
 “We need more research on when and how secondary transfer can occur.” 
 
 It is important to recognize that DNA amplification kits have become much more sensitive than they were in the past. As a result, the types of samples being analysed have expanded. Investigators no longer need to identify and request analysis of body fluids such as blood, semen and saliva. They can swab surfaces for otherwise invisible cells left behind, on the handle of a weapon or on a windowsill, perhaps, and ask labs to generate a DNA profile from them. The new kits can generate a full genetic profile of a suspect from as little as 100 picograms (trillionths of a gram) of DNA. 
 
 These subtleties are not usually explained in court. Instead, a jury is told that there is a one-in-a-quadrillion chance that the evidence retrieved from the crime scene did not come from a defendant. Naturally, the jurors assume that the defendant must have been there. 
 
 Given the power of modern forensic techniques to pull a DNA profile from a smudge of cells, secondary DNA transfer is no longer a purely theoretical risk. In California in 2013, a man called Lukis Anderson was arrested, held for four months and charged with murder after his DNA was found under the fingernails of a homicide victim. 
 
 Anderson had never met the victim and was severely intoxicated and in hospital when the man was killed. The same paramedics who took Anderson to hospital responded to the murder. Most likely, the paramedics were covered in Anderson's DNA, which they then inadvertently transferred. The charges were dropped. 
 
 Experiments in our labs, under the supervision of forensic anthropologist Krista Latham, show how easily DNA can be transferred to an object. 
 
 We asked pairs of people to shake hands for two minutes and then each individual handled a separate knife. In 85% of cases, the DNA of the other person was transferred to the knife and profiled. In one-fifth of the samples, the DNA analysis identified this other person as the main or only contributor of DNA to the 'weapon' ( et al. J. Forensic Sci. http://doi.org/8j2; 2015). 
 
 How significant is the result of a single study? Other analyses have shown that DNA transfer can be unpredictable and can depend on environmental conditions. We need more research on when and how secondary transfer can occur. 
 
 At the very least, the results highlight again that samples at crime scenes must be gathered with great care. DNA can persist on latex gloves, so they must be changed — or bleached — before and after handling evidence. 
 
 Even apparently rigorous evidence such as DNA profiles can be interpreted in multiple ways, some of which will be incorrect. As the technology to generate these profiles continues to accelerate, so must our efforts to sift out possible mistakes. ||||| A 2013 murder case shows just how dangerous it can be to rely on DNA evidence, an expert said Wednesday. 
 
 DNA found under the fingernails of murdered California millionaire Raveesh Kumra led police to Lukis Anderson of San Jose. But after Anderson spent five months in jail, investigators found out that Anderson, a homeless alcoholic, had in fact been drunk and passed out in a hospital at the time of the attack. 
 
 Forensic analyst Karen Gincoo checks a tray of evidence vials from rape kits in the biology lab at the Houston Forensic Science Center in Houston on Thursday, April 2, 2015. Pat Sullivan / AP 
 
 He couldn't have committed the murder. But investigators discovered that two paramedics who had picked up and moved Anderson also responded to the Kumra murder scene. 
 
 Police determined that DNA from Anderson somehow got transferred to Kumra's body by the paramedics. 
 
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 It's a perfect example of how modern, ultra-sensitive DNA techniques can lead police, judges and juries astray, says Cynthia Cale, a DNA analyst of both the University of Indianapolis and Strand Diagnostics. 
 
 "Research done by me and others at the University of Indianapolis in Indiana has highlighted how unreliable this kind of evidence can be," Cale writes in a letter to the journal Nature. 
 
 "We have found that it is relatively straightforward for an innocent person's DNA to be inadvertently transferred to surfaces that he or she has never come into contact with. This could place people at crime scenes that they had never visited or link them to weapons they had never handled," she added. 
 
 "We asked pairs of people to shake hands for two minutes and then each individual handled a separate knife. In 85 percent of cases, the DNA of the other person was transferred to the knife and profiled. In one-fifth of the samples, the DNA analysis identified this other person as the main or only contributor of DNA to the 'weapon'." 
 
 DNA evidence once had to be taken from bodily fluids or fairly large bits of tissue. New technology means very small amounts can be picked up and amplified in a lab to get a reading, Cale noted. 
 
 "These subtleties are not usually explained in court. Instead, a jury is told that there is a one-in-a-quadrillion chance that the evidence retrieved from the crime scene did not come from a defendant. Naturally, the jurors assume that the defendant must have been there," Cale wrote. 
 
 "We urgently need to review how DNA evidence is assessed, viewed and described." ||||| In the first decade or so of DNA testing, criminalists focused their attention on only the most obvious cases of primary transfer—on examining clear and unambiguous stains left directly by the perpetrator, such as the bloodied knife or semen-stained sheets—rather than looking for other, invisible cells left behind. It was not until 1997 that a study first suggested that DNA typing might be capable of recovering a profile from skin or other DNA-carrying cells on objects merely handled by the perpetrator. 
 
 Once DNA is deposited on a surface, be it a person or an object, how long will it stay there? 
 
 Such trace, or touch, evidence, quickly garnered great attention. Since then, a series of studies has shown that DNA transfers quite readily based on even brief amounts of contact. Moreover, the amount deposited can range from just a few cells to a sizeable amount (recall that each cell contains about 6 pg of DNA, and most ordinary DNA typing methods today return reliable results with roughly 500 pg of template). 
 
 If primary transfer is the engine that turns the wheels of justice, by identifying perpetrators based on the DNA they leave behind, then secondary transfer is the cog that causes that wheel to grind to a halt. Secondary transfer occurs when DNA from one person is transferred to a person or object, even though the person never came into contact with that other person or object. Instead, the transfer occurs via an intermediary person or object. The problem is that what looks like primary transfer, and leads investigators to believe that the suspect was there, might in fact be secondary transfer. 
 
 Returning to the earlier example, suppose you give your spouse a kiss on the cheek. Later that day, your spouse meets an old friend, who plants a kiss on the same cheek. Secondary transfer may occur: your DNA may now be on the face or lips of the friend, even if you have never seen or met that person. The same scenario can happen with an object. For instance, when you send your child off to school with the lunchbox you packed, your DNA goes to school, too, and may end up on the teacher who collects the boxes to put them away, or on the shelf where they are stored, or on the table where your child eats—even though you do not come along personally or interact with any of those persons or surfaces. 
 
 The transference is called secondary because the original depositor of the DNA did not come into direct contact with place where the DNA was found. Using ordinals can also help additional suggest degrees of remove from the initial DNA donor. For instance, it constitutes tertiary transfer if your DNA starts on the lunchbox, goes from the box to your child’s teacher, and then is passed from the teacher to her aide. Tertiary helps signify that the DNA ended up in a place three steps removed from the initial contact with its owner. The ordinals used in the forensic context tend to start from the original source of the DNA sample, always defining its journey in reference to that originator. 
 
 But this language is also a bit confusing. DNA that transfers between persons or objects away from the originator is also, in a sense, directly transferring. Imagine that a source leaves a pool of blood on the floor. I step in it, which transfers that blood to the bottom of my shoe, which in turn rubs onto to a rung of a chair where I prop up my feet, which then transfers to the pants leg of a person who later sits in that chair. Each of those contact moments was direct, albeit at increasing remove from the original DNA depositor. Or if I speak, and tiny bits of saliva deposit my DNA on the ground in front of me—or I do my laundry and the mingling of the clothes distributes the shed cells contained therein—in both cases the transfer is direct. But, in the saliva example, the transfer is primary because the DNA emanated directly from the source, whereas in the laundry example it is secondary because it is moving among objects with which I initially came into contact. 
 
 Another important concept to grasp when considering transfer is the notion of persistence. Once DNA is deposited on a surface, be it a person or an object, how long will it stay there? How easily might it be erased or overwritten by contact from a subsequent person? Is the last person to come into contact with a person or item always the one to leave the dominant DNA profile in a sample? Such questions can be further complicated by asking whether the quality and duration of contact by each person influences the amount of transference to or from each party in the chain. 
 
 The Yale story illustrates how important it is, now that we have the capacity to link perpetrators to an offense by the presence of their DNA in criminal evidence, to understand completely how easy it is for such cells to appear in a sample with which the DNA donor has never come into contact. Absent a good alibi—in this case, the irrefutable proof of his prior death—the worker might have ended up implicated in the crime. His familiarity with the space, along with his prior record, might have been used against him to prove that he had special knowledge of a good place to dispose of the body. Unfortunately, we still know very little about DNA transfer, and one major entity that funds DNA studies—the National Institute of Justice—has not expressed much interest in learning more.
– DNA testing has improved to the point where it is starting to incriminate the wrong people in some cases, experts warn. The problem is super-sensitive equipment and DNA transfer, which can lead to a person's DNA being found on a body or murder weapon even if they had only shaken hands with the real criminal or visited the crime scene, reports NBC News, quoting a column from DNA analyst Cynthia Cale in the journal Nature. New DNA kits can identify a person from even the tiniest scraps of biological material, Cale writes, but when juries are told there is a "one-in-a-quadrillion chance that the evidence retrieved from the crime scene did not come from a defendant," they aren't usually told about the possibility of transfer, she writes. Cale—who says we "urgently need to review how DNA evidence is assessed, viewed, and described"—cites the case of an innocent man who spent months in jail after his DNA was found on a murder victim, having been transferred there by paramedics who took the man to a hospital before going to the murder scene. At the New Republic, law professor Erin Murphy describes how DNA transfer initially led police to an impossible suspect in the 2009 murder of Yale student Annie Le. After her body was found in her lab's crawl space, DNA found on her skin and underwear matched that of a convicted offender in the area. He had left DNA behind when he carried out construction work in the lab, Murphy writes, but he might still have gone to prison for the crime—if he hadn't died two years earlier.