A drop of blood left by a suspect at a crime scene is a treasure trove for forensic scientists. Genetic information extracted from such biological samples can be compared against DNA databases to see whether a sample’s DNA sequence is a match for any known offenders, for example. To protect individuals’ privacy, these analyses, known as DNA fingerprinting, are normally restricted to parts of the genome not involved in creating proteins. But in some countries, investigators hoping to narrow down their pool of suspects are allowed to identify certain protein-coding sequences that can help predict skin or eye color . And soon, scientists may be able to find out even more from an offender’s DNA—including their age.

A new forensic approach analyzes the chemical tags attached to DNA, rather than genetic sequences themselves. These molecules, which can switch genes on and off, get added onto DNA throughout our life span in a process called DNA methylation. And because the patterns of DNA methylation change as we age, they could provide a good indication of how old a suspect is.

But this technique could inadvertently reveal a lot more about a suspect’s health and lifestyle , raising tricky legal and ethical questions that may demand new privacy safeguards, scientists suggest in a commentary in the July issue of Trends in Genetics . Science talked to two of the authors, Mahsa Shabani and Bram Bekaert of the University of Leuven in Belgium, who say that when it comes to this new world of forensics, “We have to just make sure that we’re not breaching any ethical boundaries, because we can get a lot more information than people actually realize at the moment.” The interview has been edited for length and clarity.

Q: How did you get interested in the use of DNA methylation in forensics?

Bram Bekaert: We saw that [clinical researchers] were able to associate DNA methylation patterns to chronological age, and extract some additional clinical information. And we started to develop our own simple assays … to estimate age for forensic purposes.

Mahsa Shabani: We realised that this actually creates some potential for extracting more personal data, and this raised some ethical and legal concerns.

Q: What kind of personal data?

B.B.: There was one paper that recently came out where forensic scientists who had developed an age-prediction assay were actually showing they could determine whether individuals had chronic lymphocytic leukemia. That’s just a single example—methylation has also been associated with risk of mortality and all kinds of other pathologies.

Q: Does a criminal who leaves behind DNA still have a right to protect their personal information?

M.S.: This was one of the questions we were raising. If this method leads us to extract more and more information, what should be our criteria—when should we stop? You may argue that in this specific context any information that could help you to narrow down the [suspect] pool and find the “donor” of the samples … should not create any problem. But on the other hand … previous regulations have always been more restrictive in terms of what you can do with DNA samples.

B.B.: DNA methylation is in part heritable … so that’s something we have to take into account as well. It’s not just about the privacy of the individual himself … it’s also about his son, his daughter, or his parents or his siblings.

Q: In many countries, regulations prevent the use of DNA samples to predict identifiable information like hair or skin color. Why don’t those laws apply to this new method?

B.B.: We are not actually looking at DNA sequences, so the law does not apply to this type of technology. We’re looking at the chemical compounds on top of the DNA sequence—that’s still what’s missing in the law. Right now, I can just look at all of these markers, without breaking the law, and determine whether somebody has cancer or not. So [the technology] has to be regulated or controlled in some kind of way.

Q: What could that regulation look like?

B.B.: We could limit our assay to just markers that reveal information on the chronological age and do not contain any information on medical issues. So markers that have been associated with cancer incidence should be left out—that would be one solution. The problem with that is that in order to get fairly high [age] accuracy you need more markers.

The second approach could be to restrict the information that [a forensic scientist can give] to the police and to the magistrates, so you only provide the predicted age, based on a sample’s methylation values. In my opinion that would probably be the best solution.

Q: Aside from age, what other useful information could DNA methylation tell forensic scientists?

B.B.: Because DNA methylation is cell type–specific, we can use the patterns to determine what kind of cell we’re dealing with. For example, if I want to make sure that the stain that I’ve recovered from a crime scene is blood, I can use DNA methylation patterns … and you can do so for all kinds of bodily secretions and types of cells.

Q: Are these tests being used in police investigations already?

B.B.: Not really—most of them are still in the research stage, because when you want to use a forensic test it has to be validated and accredited. But they will come—without a doubt they will come—because police are really, really interested in those kinds of tests.

Q: Are you optimistic that they will turn out to be useful forensic tools?

B.B.: I’m very convinced that these kinds of tools do have their purpose. … We both support these kinds of tests, we have to just make sure that we’re not breaching any ethical boundaries, because we can get a lot more information than people actually realize at the moment.