Transfer DNA: An Updated, Unbiased Term for Touch/Trace DNA

by Angela Williamson, Abigail Bathrick, Mike Cariola, Bode Technology

Forensic scientists have long recognized the phenomenon and probative value of DNA transferred to people or objects through direct or indirect contact¹. Various terms have been utilized by the forensic community over the past decades to include Touch, Trace, and Wearer DNA. However, the need for standardized terminology still exists in order to avoid any implicit bias by forensic analysts with regards to the exact biological origin, amount, and method of transfer of this DNA. This is especially important when considering the interpretation and weight of forensic evidence by jurors. Current terminology may result in laypersons assigning too much, or too little, value to forensic evidence. Herein, we will explore the most commonly used terms and their associated issues.

In order to prevent any ongoing confusion or biases within the scientific and legal community, we propose the use of "Transfer DNA" to serve as an all-encompassing term for DNA that has transferred from one surface/person to another, and where the biological source and mechanism of deposition cannot be definitively attributed.

Is the term Touch DNA out of touch?

Since the mid-2000s, Touch DNA has been commonly used as the term for DNA that is assumed to have been left behind from skin cells when a person touches or comes into contact with an item/individual². However, the use of the term Touch DNA has become increasingly contentious. Touch DNA can be viewed as biased and misleading, especially to a jury, since it is perceived as implying direct contact between an individual and another person or object. Because of this, courts are increasingly discouraging the use of the term Touch DNA. In the case of State v. Phillips, the conviction was reversed and the case remanded for a new trial, in part, because transfer DNA evidence was presented in a way that led jurors to infer contact and suspect involvement that was not science-based³. In the State of Idaho v. Bryan C. Kohberger for the Idaho Four murders, a defense motion in limine led the judge to instruct lawyers and experts to avoid phrases like Touch, Trace or Contact DNA to prevent juror confusion⁴. These rulings show that courts recognize the risks of using terminology implying actions that DNA evidence cannot confirm, along with reflecting legitimate concerns that jurors may unknowingly treat the presence of DNA as proof of contact, timing, or intent.

Within the forensic community, it is also widely known that direct contact isn’t always the mechanism for deposition of this DNA, and newer studies have expanded upon the occurrence of secondary and tertiary transfer resulting in detectable DNA profiles⁵.

Secondary transfer occurs when DNA is transferred from one person to another through skin-to-skin contact, such as shaking hands, and then to an object. Tertiary transfer involves a more complex process where DNA is transferred from an individual to an object, then to a vector (like a hand), and finally to another object/person⁶. Both types of transfer are important in forensic science, as they can affect the detection and interpretation of DNA evidence. One of the factors that influence secondary and tertiary transfer is the amount of DNA shed by different individuals (which cannot be predicted or measured). Other influencing factors are the type and duration of contact between individuals or objects, the nature and quality of the DNA source, along with exceptions and variations depending on individual characteristics and environmental conditions⁷. Studies have also shown how DNA can transfer between items in forensic evidence packaging⁸.

Hence, under most circumstances, a forensic DNA analyst cannot definitively infer how the DNA came to be deposited on the person or evidence in question, negating the use of the term Touch DNA. Retiring Touch DNA in favor of a neutral, standardized term that describes DNA detection without implying action would promote scientific accuracy, help provide clearer testimony, and fairer trial outcomes.

The case against the term Trace DNA

Trace DNA has long been used as a synonymous term to Touch DNA9, but this presents its own issues, mainly centered on the fact that Trace typically means "a minute and often barely detectable amount or indication"¹⁰. Trace DNA is further defined by the National Institute of Justice (NIJ) as ‘small amounts of DNA left behind at a crime scene, such as a few cells left on objects’¹¹. Implying that Touch/Trace DNA is typically low-level is misleading given that these types of samples can vary greatly in the amount of DNA deposited and detected¹². For example, in the 2022 Idaho Four murders, a knife sheath recovered from the bed of one of the victims was swabbed for transfer DNA on several areas, including the entire leather portion of the strap (top and bottom) and the underside of the button. During court proceedings it was revealed that the sample contained sufficient DNA to generate both a Combined DNA Index System (CODIS) and Single Nucleotide Polymorphism (SNP) profile for Forensic Investigative Genetic Genealogy (FIGG).^13,14

In a recent Transfer DNA sampling method internal validation study, a simulated assailant handled typical crime scene items, including zip ties, rope used as mock ligature, t-shirt, jeans and underwear. Sampling of all items for Transfer DNA resulted in 100% complete profiles for the simulated assailant. The jeans sample generated a total yield of over 30 ng of DNA for both the simulated victim and assailant, demonstrating that DNA left behind through direct contact (commonly encountered in violent crime cases) is not consistently in low amounts or challenging to profile. Similar results have also been reported in numerous other transfer DNA studies throughout the past 20 years¹².

What about Wearer DNA?

"Wearer DNA" is typically the accepted term when referencing skin cells and/or sweat left behind by someone who last wore an item (e.g., inside of a baseball cap, armpits of a shirt). This type of DNA can be crucial in forensic investigations, especially when clothing items thought to belong to the perpetrator are left behind at a crime scene¹⁵. However, given unknown variables such as laundering, environmental conditions and shedder status, it is impossible to state that the major DNA contributor detected during targeted sampling is definitively the person who last wore the item. Like Touch DNA, Wearer DNA also implies direct contact and an action by the individual.

Aligning language with evidence – why Transfer DNA works

Unlike Touch, Trace, and Wearer DNA; Transfer DNA does not imply any specific level of DNA, nor does it imply direct or indirect deposition of DNA on a person or object. It simply indicates that DNA was transferred to a surface of interest within the context of a criminal matter.

We encourage major forensic organizations to adopt the term Transfer DNA for official laboratory and legal proceedings. We feel confident that this term will satisfy all stakeholders within the criminal justice community, along with providing less confusion for judges and jurors who serve as the ultimate decision-makers in assigning value to forensic DNA evidence.

References

1. van Oorschot RAH and Jones MK. DNA fingerprints from fingerprints. Nature. Vol 387 (1997) 767.
2. Williamson AL. Touch DNA: Forensic Collection and Application to Investigations. J Assoc Crime Scene Reconstr. 2012:18(1);1-5.
3. https://caselaw.findlaw.com/court/sc-supreme-court/2069063.html
4. 041825+Order+on+Defendants+Motion+in+Limine+RE+Rylene+Nowlin+and+Touch+and+Contact+DNA.pdf
5. Sessa F, Pomara C, Esposito M, Grassi P, Cocimano G, Salerno M. Indirect DNA Transfer and Forensic Implications: A Literature Review. Genes (Basel). 2023 Nov 28;14(12):2153. doi: 10.3390/genes14122153. PMID: 38136975; PMCID: PMC10742555.
6. McCrane SM, Mulligan CJ. An innovative transfer DNA experimental design and qPCR assay to identify primary, secondary, and tertiary DNA transfer. J Forensic Sci. 2024;69:618–30. https://doi.org/10.1111/1556-4029.15444
7. Goray, M et al. Investigation of secondary DNA transfer of skin cells under controlled test conditions. Legal Medicine 12 (2010) 117-120.
8. Goray, M et al. DNA transfer within forensic exhibit packaging: Potential for DNA loss and relocation, Forensic Si. Int. Genet. (2011), doi 10.1016j.fsigen.2011.03.013
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10. TRACE Definition & Meaning – Merriam-Webster
11. National Institute of Justice, “Improving Analysis of “Trace DNA” Evidence ,” January 16, 2024, nij.ojp.gov:https://nij.ojp.gov/topics/articles/improving-analysis-trace-dna-evidence
12. van Oorschot RAH, Szkuta B, Meakin GE, Kokshoorn B, Goray M, DNA transfer in forensic science: a review, Forensic Science International: Genetics (2018), https://doi.org/10.1016/j.fsigen.2018.10.014
13. https://coi.isc.idaho.gov/docs/CR01-24-31665/2025/022125-Transcript-Redacted-hearing-held-Jan23-2025.pdf
14. https://www.ci.moscow.id.us/1165/Kohberger-Investigation-Documents
15. Magee AM, Breathnach M, Doak S, Thornton F, Noone C, McKenna LG. Wearer and non-wearer DNA on the collars and cuffs of upper garments of worn clothing. Forensic Sci Int Genet. 2018 May;34:152-161. doi: 10.1016/j.fsigen.2018.02.011. Epub 2018 Feb 13. PMID: 29482105.