A DNA mixture arises whenever several people have deposited biological material on the same trace. The resulting profile is difficult to interpret, particularly when one of the contributors is present at a very low level. A review published on 19 October 2026 in the International Journal of Legal Medicine examines an alternative approach, in which cells are isolated before they are analysed [1]. Because each cell carries the DNA of only one person, a separate profile can be generated for each contributor. The method is advancing, but it is constrained by the minute amount of DNA in a single cell and is not yet ready for routine casework.
DNA mixtures are an everyday challenge
Biological traces recovered from a crime scene often contain DNA from more than one person. At the Lyon forensic science laboratory, now part of the French National Forensic Science Service (SNPS), mixtures already accounted for more than 30% of the profiles analysed in 2017 [2]. In Denmark, a nationwide study found that around two thirds of the profiles reported between 2016 and 2024 were mixtures [3]. Interpretation becomes harder as the number of contributors rises, or when one of them has left far more biological material than the others. The profile of the minor contributor may then be masked.
Forensic laboratories have two tools for dealing with mixtures. Differential extraction separates sperm cells from epithelial cells in samples collected after a sexual assault. It is of no use when all the cells are of the same type, as with blood from two people or semen from two men [7]. Probabilistic genotyping software then assists with the interpretation of the mixture, but its output depends on the quantity and quality of the DNA available. Some profiles remain uninterpretable.
Separating cells before DNA analysis
Ruijia Liu, Zheng Wang and Jing Liu, of the Institute of Forensic Medicine at Sichuan University, review the techniques used to isolate cells one by one [1]. Each cell contains the DNA of a single individual. When cells are processed together, their DNA is pooled and yields a mixed profile. When they are separated beforehand, each cell yields the profile of a single contributor. Separation must take place before cell lysis, that is, before the DNA is extracted from the cell. A portion of the sample that has not yet been processed is therefore required [2].
Four families of techniques are available:
- Micromanipulation picks up cells one at a time under a microscope. It is precise but slow, and requires a trained operator.
- Laser capture microdissection uses a laser beam to detach selected cells from their substrate. It offers the same advantages and has the same constraints.
- Flow cytometry detects cells by means of a fluorescent label and sorts them automatically in large numbers.
- The DEPArray system moves cells using electric fields. The operator views them on screen and selects those to be analysed. Its cost and the small number of cells processed per run still limit its uptake [1, 3].
A single cell is almost never enough
A diploid human cell contains only about six picograms of DNA, roughly a hundred times less than the amount a laboratory normally analyses [9]. With so little template, amplification often leaves gaps in the profile because some alleles go undetected (allelic drop-out). The slightest contamination can also introduce alleles that do not belong to the cell (drop-in). A forensic scientist who fails to account for these two effects risks misinterpreting the profile.
A DEPArray validation study published in 2024 puts figures on this limitation. A single diploid cell analysed on its own recovers only 80% of the profile on average. A sperm cell recovers about half, since it carries only one copy of each chromosome. A full profile becomes achievable from three diploid cells under the most favourable conditions, whereas around fifteen sperm cells are needed to exceed 90% of a man’s profile [3, 4]. Laboratories therefore group the profiles of several cells that appear to originate from the same person and build a consensus profile. In practice, the term “single cell” refers to an analysis based on a small number of cells [3].
These data are then evaluated using probabilistic models [6]. The team of Catherine Grgicak, at Rutgers University, and Ken Duffy tested this approach on 630 laboratory-prepared mixtures with up to five donors. For 91% of cell clusters, the likelihood ratio in favour of the true contributor exceeded one billion billion (1018) [5]. These mixtures were, however, prepared from known donors, and there is no guarantee that the same results would be obtained with casework traces, which may be aged or degraded.
What the method cannot tell us
Isolating the cells in a biological trace makes it possible to generate a separate DNA profile for each contributor and to compare it with that of a given person. The analysis therefore addresses the source of the DNA. It says nothing about how or when that DNA was deposited. Questions of transfer and persistence remain open and must be answered with other data. An opinion review published in June 2026 in the World Journal of Biological Chemistry stresses this point [3]. It was written by a team of physicians and biologists working in Greece and Cyprus, outside forensic laboratories, who draw together recent validation studies. Their conclusion is cautious. Single-cell analysis should complement conventional analysis in carefully selected cases, not replace it. Before applying it to casework, a laboratory should have measured its contamination rate, the minimum number of cells required for a consensus profile and the probability of recovering at least one cell from the minor contributor.
CapCell, a European project to be presented in Paris
The topic will be discussed in Paris on 20 November 2026 at the annual conference of the CEJC (Compagnie des Experts de Justice en Criminalistique), devoted to time in forensic science. Camille Ropert, a 2025 CEJC award winner, will present the European project CapCell. Coordinated by Athina Vidaki at Maastricht University, CapCell brings together thirteen partners from eight countries, including the Netherlands Forensic Institute (NFI), the Estonian Forensic Science Institute and the Austrian Federal Ministry of the Interior. It has a budget of about 4.5 million euros for the period from October 2025 to September 2029. Its aim is to isolate the perpetrator’s cells in samples collected after sexual violence, using mobile microfluidic devices, and then to validate these tools in laboratories and police services [8].
Sources
[1] Liu R., Wang Z., Liu J., « Single-cell strategies for the interpretation of complex forensic DNA mixtures: isolation techniques, genotype data integration, and prospects for application », International Journal of Legal Medicine, 2026. https://doi.org/10.1007/s00414-026-04046-y
[2] Laurent F.-X., Vibrac G., Rubio A., Thévenot M.-T., Pène L., « Les nouvelles technologies d’analyses ADN au service des enquêtes judiciaires », médecine/sciences, 2017, 33(11), 971.
[3] Evangelou K., Angeli P., Polydorou A., Petropoulou T., « Single-cell deoxyribonucleic acid typing for forensic mixtures and trace evidence: Opportunities, validation requirements, and reporting limits », World Journal of Biological Chemistry, 2026, 17(2), 121467. https://doi.org/10.4331/wjbc.v17.i2.121467
[4] Schulte et al., « DEPArray™ single-cell technology: A validation study for forensic applications », Forensic Science International: Genetics, 2024. https://www.fsigenetics.com/article/S1872-4973(24)00020-6/fulltext
[5] Grgicak C.M., Bhembe Q., Slooten K., Sheth N.C., Duffy K.R., Lun D.S., « Single-cell investigative genetics: Single-cell data produces genotype distributions concentrated at the true genotype across all mixture complexities », Forensic Science International: Genetics, 2024, 69, 103000. https://doi.org/10.1016/j.fsigen.2023.103000
[6] Bhembe Q., Lun D.S., Duffy K.R., Grgicak C.M., « Resolving the Forensic DNA Mixture Problem through the Development of End-to-End Single-Cell Pipelines », Forensic Science International: Genetics, 2025, 81.
[7] Huffman K., Ballantyne J., « Single cell genomics applications in forensic science: Current state and future directions », iScience, 2023, 26(11), 107961. https://doi.org/10.1016/j.isci.2023.107961
[8] Projet CapCell, fiche CORDIS n° 101225737. https://cordis.europa.eu/project/id/101225737/fr
[9] Huffman K., Hanson E., Ballantyne J., « Cell Subsampling Recovers Probative DNA Profile Information from Unresolvable/Undetectable Minor Donors in Mixtures », Genes, 2022, 13(7), 1117. https://doi.org/10.3390/genes13071117
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