This post is inspired by Ken Mason's Forensic Medicine for Lawyers that rekindled the author’s interest in forensic science and medicine: an influence which continues to direct her research interests.
In his 2014 Kalisher lecture, the Lord Chief Justice expressed concerns regarding the potential of new, or complex, scientific developments to tarnish the status of forensic DNA profiling. Lord Thomas identified the 'challenge for all…advocates and judges – to manage the presentation and testing of forensic evidence in such a way as to avoid fatally undermining confidence.’ The judiciary’s concern to maintain public confidence in this forensic keystone is well founded, given the unparalleled discriminatory potential of DNA profiling techniques, and their unique ability to link the bio-identity of suspect populations to an ineradicable bodily substrate. Indeed, a glance at recent news stories would suggest that the LCJ’s challenge has been met, and that the reputation of DNA profiling – as the ‘gold standard’ of forensic identification tools - remains secure.
Nonetheless, the persistent presence of these overarching concerns may go some way to explain the decision in R v Tsekiri  EWCA Crim 40 . In this case the Court of Appeal of England and Wales addressed the problematic issue of DNA ‘transfer and persistence’. The facts are as follows: In June 2016, a Ms. Carr unlocked her car. At this point, an unidentified man attempted to enter the vehicle. A brief struggle ensued, and the man escaped, with Ms. Carr’s gold necklace. Swabs were taken from the exterior door handle of the vehicle and these revealed a mixed DNA profile. The profile belonging to the major contributor was consistent with that taken from the appellant (with a match probability of 1:1 billion). There was at least one other minor contributor. The reporting scientist could reach no determination as to when the major components of the mixed DNA profile had been deposited, nor their source. The deposit could have been due to that person touching the door handle, or due to indirect secondary transfer through an intermediary (though the reporting scientist considered this unlikely, given that the DNA in question was the major contributor to the profile). The appellant submitted that there was no case to answer given the dearth of evidence.
Held (Lord Thomas, LCJ): Where a defendant's DNA profile - recovered at the scene - is the only evidence, that evidence is sufficient to raise a case to answer, provided the match probability is in the order of 1:1 billion. Sufficiency will depend on the facts of the case. The court provided a non-exhaustive list of relevant factors to be taken into account:
-Can the presence of the DNA evidence be otherwise accounted for?
-Was the article apparently associated with the offence itself?
-How readily movable was the article in question?
-Is there evidence of some geographical association between the offence and the offender?
-In the case of a mixed profile, is the DNA profile which matches the defendant, the major contributor?
-Is it more or less likely that the DNA profile attributable to the defendant was deposited by primary, or secondary, transfer?
The court stressed the crucial point, ‘that there is no evidential or legal principle which prevents a case solely dependent on the presence of the defendant's DNA profile on an article left at the scene of a crime being considered by a jury.’
Whilst the court’s decision may help to maintain confidence in the probative value of forensic DNA profiling techniques, it may be argued that the judiciary have purchased security at the price of accuracy. The case highlights a growing problem for DNA profiling experts, whose technologies are now so sensitive that they routinely report ‘mixed’ samples, including deposits made indirectly through intermediate contacts, and low-template DNA shed naturally by individuals and spread through contact and dispersal. In many cases, scientists can de-convolute mixed samples using their expertise, often with the aid of complex computer algorithms. The de-convoluted results may reveal a major contributor, and one - or more - minor contributors, all of whose profiles may be complete, or partial.
However, the evaluative problems are not limited to attribution. Whilst individual profiles may be ‘matched’ to individuals on the DNA database - with probabilities in the order of 1:1 billion - the resulting evidence may be effectively neutralised by the issue of the transfer and persistence of DNA deposits across multiple surfaces. The interposition of such issues shifts the focus of analysis, such that the courts should no longer address themselves to the question of ‘to whom does this DNA sample belong?’ The salient question becomes: ‘how did this DNA sample come to get here?’
Given that different individuals shed epithelial ‘touch’ DNA at different rates; that different surfaces retain DNA at different rates; and that DNA may be deposited by secondary, or even tertiary, intermediaries (not to mention the presence of environmental factors); the task of answering activity propositions with any degree of certainty may prove difficult, if not impossible.
The problem, then, is encapsulated in this excerpt from a research interview with a lead DNA profiling scientist. She considers a hypothetical situation based on the analysis of a profile recovered from a pair of gloves left at the scene of a robbery. The suspect claims that the gloves are his, but that they were borrowed by another individual:
The above concerns are supported by laboratory studies, which demonstrate that DNA, deposited on an object as a result of secondary transfer, is frequently identified as either the only contributor, or the major contributor, to a mixed profile, despite the contributing individual never coming into direct contact with the object. 
Such studies illustrate the risks of assuming that DNA, recovered from an object, results from a direct contact. They also cast doubt on the Court of Appeal’s approach to such issues, given that the latter are effectively attempting to weight transfer and persistence. Indeed, further questions are raised as to whether it was possible for the court in Tsekiri to arrive at strong assertions, such that ‘there can be no doubt that the offender did touch the article in question.’ Further, studies would appear to confound the bare proposition that ‘secondary transfer was an unlikely explanation for the presence of the appellant's DNA on the door handle.’ Nor does the connection between the suspect and a particular geographical location necessarily aid determination, since such a connection may be equally supportive of secondary transfer scenarios.
The failure of the courts to take this opportunity to address the pressing problem of DNA transfer and persistence is perhaps understandable, given that the evidence in the above case was not subject to rigorous cross-examination. However, both practitioners and the courts should be aware of the issues involved, alive to the importance of context, and wary of attempts to collapse the question of ‘how’ into the less scientifically problematic question of ‘who?’
Whilst there may be a number of economic, policy, and procedural reasons for such an approach - centering on the perceived necessity to maintain the unblemished status of forensic DNA - attempts to evade the pressing question of transfer and persistence may provide only temporary security for accused persons and society alike.
 R v Tsekiri  EWCA Crim 40 (17 February 2017) ;
 The 2014 Criminal Bar Association Kalisher Lecture: Expert Evidence: The future of forensic science in criminal trials. The Right Hon. The Lord Thomas of Cwmgiedd. 14th October 2014
 R v Tsekiri  EWCA Crim 40 at 21
 Taroni F., Biedermann A., Vuille J., Morling N. (2013). Whose DNA is this? How relevant a question? (a note for forensic scientists). Forensic Sci. Int. Genet. 7, 467–470
 Despite rigorous attempts to provide coherent data in respect of DNA transfer and persistence, research is sporadic, and results lack meaningful generalisability. See, for example: C. Davies, et al., Assessing primary, secondary and tertiary DNA transfer using the Promega ESI-17 Fast PCR chemistry, Forensic Sci. Int. Gene. Suppl. (2015)
 Interview with DNA lead scientist (Tier 2 forensic science provider). Oxford, 2015.
 Cale CM, Bush GL, Earll ME & Latham KE (2016) Could Secondary DNA Transfer Falsely Place Someone at the Scene of a Crime? Journal of Forensic Science, January 2016;61(1): pp.196-203.