Eleanor Alison May Graham, 1 Ph.D. and Guy Nathan Rutty, 1 M.D. Investigation into ‘‘Normal’’ Background DNA on Adult Necks: Implications for DNA Profiling of Manual Strangulation Victims ABSTRACT: Others have investigated the role that DNA profiling could play as a method for identifying the perpetrator of manual strangula- tion. These studies have demonstrated that it is possible to collect offender DNA from the skin surface of a victim following physical contact. It is not known whether nonself biological material is normally present on the skin surface due to adventitious transfer occurring during innocent everyday interactions. To test the hypothesis that detectable amounts of nonself DNA are normally present on the skin surface of healthy adult individuals due to the adventitious transfer of DNA occurring during normal day-to-day social interactions, we designed an experiment in three phases. Phase 1 was used to deduce which DNA collection, extraction, and amplification methods were suited to investigating this question. During phase 2, the neck sur- face of 24 healthy adult volunteers was swabbed. DNA was extracted using the QIAamp DNA mini kit and amplified using the SGM Plus PCR amplification kit, using 28 PCR cycles. The work carried out during phase 3 involved a simulated assault to investigate primary and secondary trans- fer of DNA during physical contact. It was found that 23% of neck areas swabbed during phase 2 of this investigation showed nondonor alleles in the resulting DNA profile, with 5% of areas showing six or more nondonor alleles. The results of phase 3 showed that primary, secondary, and zero transfer of victim and ⁄ or offender DNA could be observed after physical contact and that alleles from an unknown source could still be detected in this more controlled experiment. The data presented in this paper demonstrate that DNA profiles generated after swabbing the skin surface of healthy adults can include components of an unknown source, present due to adventitious transfer. These components, if present in large quantities, have the potential to interfere with DNA profile interpretation of swabs taken for the investigation of physical assault by DNA profiling. KEYWORDS: forensic science, DNA, strangulation, manual, normal, offender, victim Wiegand and Kleiber (1) and Rutty (2) investigated the role that DNA profiling could play as a method for identifying the perpetrator of manual strangulation. Both these original studies demonstrated that it is possible to collect the offender DNA from the skin surface of a victim following physical contact. Both experiments were however carried out under controlled conditions, with sampled areas being specifically washed before initiation of each transfer experiment. The possibility that a DNA profile could arise from a source other than the two indi- viduals involved in each experiment was discussed by Rutty. He proposed that adventitious DNA transfer occurred prior to or after the experimental transfer experiment resulting in the deposi- tion and possible transfer of third party DNA onto the sampled areas of both offender and victim. The existence of secondary transfer is perhaps the most controver- sial and least understood area of forensic DNA profiling. The poten- tial problem was first reported by van Oorschot and Jones (3). They described that substantial DNA transfer could occur during initial contact, and that objects handled by numerous individuals produced mixed DNA profiles, with the most prevalent DNA profile not always arising from the last individual to handle each particular object. It was quickly realized that persistence of DNA sources from numerous individuals on inanimate objects could potentially hinder DNA profiling of trace evidence, such as fingerprints, by resulting in DNA mixtures, including components of innocent third parties (4). Although secondary transfer was not observed during the inves- tigations of Ladd et al., it was again observed that DNA deposition on animate and inanimate objects appeared to be dependent upon the individual tested. In an attempt to address these conflicting results, Lowe et al. initiated a new series of laboratory controlled experiments, resulting in the concept of shedder status (4,5). The shedder status of an individual accounts for the difference in an individual’s ability to deposit their own DNA onto an object that both van Oorschot and Ladd had previously commented upon. We hypothesize that detectable levels of nonself DNA are nor- mally present on the skin surface of healthy adult individuals due to the adventitious transfer of DNA that occurs during normal day- to-day social interactions. However, to date, the background level of nonself DNA present on adult neck or finger pad surfaces has not been investigated. A study was undertaken to investigate whether background levels of nonself DNA can be detected using the standard DNA collection and STR profiling methods currently practiced within the U.K. The potential implications that high levels of nonself background DNA could have on forensic investigation of physical neck assault are discussed. Materials and Methods Local ethical permission was granted for the collection of biolog- ical material from nonvulnerable, adult volunteers (LREC: 6940). Prior to swabbing, each volunteer was asked to complete a ques- tionnaire, providing details of the immediate history of the swabbed area including time since washing the area and possible sources of adventitious DNA transfer onto the swabbed area. In phases 2 and 3, the epithelial shedder status of each volunteer was determined by the method described by Lowe et al. (5). A buccal swab was collected from each volunteer for production of a reference DNA profile. Finally, as manual strangulation involves the potential application of finger pads to both the front and back of the neck, both anatomical areas were considered in this study. 1 Forensic Pathology Unit, University of Leicester, Robert Kilpatrick Building, Leicester Royal Infirmary, Leicester LE2 7LX, U.K. Received 23 June 2007; and in revised form 9 Dec. 2007; accepted 22 Dec. 2007. J Forensic Sci, September 2008, Vol. 53, No. 5 doi: 10.1111/j.1556-4029.2008.00800.x Available online at: www.blackwell-synergy.com 1074 Ó 2008 American Academy of Forensic Sciences