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Forensic Science Communications - July 2004

Forensic Science Communications - July 2004

July 2004 - Volume 6 - Number 3

Case Report

DNA from a Computer Keyboard

Ana Castello

Mercedes Alvarez
Assistant Professor of Legal Medicine

Fernando Verdu
Professor of Legal Medicine
Department of Legal Medicine
College of Medicine and Odontology
University of Valencia
Valencia, Spain

Abstract | Case | Materials and Methods | Results | Discussion | References


This article reports on samples taken from trace evidence (i.e., hair and skin) found in a computer keyboard. The purpose was to determine whether more than one person had used the computer. A commercial kit was used for the extraction and purification of DNA and was found to be effective. Three STR loci were amplified (D18S535, D1S1656, and D10S2325) using polymerase chain reaction. The results indicated the existence of DNA in the samples from more than one person.


A pharmaceutical company suspected that sensitive data was taken from a computer authorized to only one user. Because of the importance of the information in the computer, an investigation was opened. To prove whether anyone other than the authorized user had used the computer, the investigators proposed that DNA analysis be performed on the trace evidence found in the keyboard. Their hypothesis was that although the keyboard might have been cleaned or someone had used gloves, hair or skin may have fallen into the keyboard and may be a biological indicator of who had used the computer.

DNA analysis on a computer keyboard in an open-access site would be very complex because of contamination and generally should not be considered for forensic casework. However in this case, the authorized user’s workplace was in a maximum-security area. Only the authorized user could enter the space where the computer was located; therefore, the possibility of finding foreign DNA was minimal.

Materials and Methods

The possibility of the authorized user carrying DNA from another person was considered (Rutty 2002; Rutty et al. 2000). To test for this possible contamination, the original keyboard (Keyboard 2) was sent to the laboratory. The authorized user was given a new keyboard (Keyboard 1) to be used under the same conditions as the old one. Keyboard 1 was stored unless the authorized user was working on it. Control samples were obtained from Keyboard 1 to determine if the authorized user contaminated it.

Questioned Sample Preparation

From Keyboard 1 (control) and Keyboard 2 (problem) two different sample sets were obtained.

Sample A was the trace evidence recovered after turning Keyboard 1 (control) and Keyboard 2 (problem) over and shaking them gently. The trace evidence was weighed and divided into four equal samples. Each sample was placed in a sterile 1.5mL test tube and labeled Sample A1, Sample A2, Sample A3, and Sample A4.

Sample B1 was obtained by brushing a wet swab (sterile distilled water) over Keyboard 1 and Keyboard 2. The wet swabs were air-dried. Sample B2 was obtained by brushing a dry swab over the keyboards (Sweet et al. 1997). All the swabs were sealed in separate sterile 1.5mL tubes and stored at 20ºC.

Known Sample Preparation

Sample C was 200μL of recently extracted blood from the authorized user.

Processing Procedure

    • DNA extraction and purification was performed using the DNA high pure template preparation kit (Roche Diagnostics, Barcelona, Spain).
    • DNA quantification was performed by electrophoresis in an 0.8% agarose gel and ethidium bromide stain.
    • DNA typing was performed on10μl of extracted DNA. The D18S535, D1S1656, and D10S2325 loci were amplified using the following protocol: 94ºC-1min, 61ºC-1min, 72ºC-1min; 30 cycles. The amplified alleles were resolved by high-resolution polyacrilamide gel electrophoresis according to previously described methods (Lareu et al. 1998A; Lareu et al. 1998B; Wiegand et al. 1999).


The kit enabled a sufficient quantity and quality of DNA to be extracted. The amounts were from different samples fluctuating between 0.2 and 1ng/μl. The results are shown in Table 1 and Table 2. From the data in the tables, the following can be deduced:

Keyboard 1 (control)

  • STR profiles were the same for the DNA recovered from Keyboard 1, Keyboard 2, and the DNA recovered from the known Sample C.

Keyboard 2 (problem)

  • In Samples A, all three loci were consistent with the authorized user’s, but STR profiles that did not belong to this user were also discovered. This did not occur in Samples B, which were consistent with the results obtained from Sample C. Sample A results showed the presence of more than one person’s DNA. The authorized user’s DNA was present in Samples A, B, and C. These results could demonstrate that someone other than the authorized person used the computer.

Table 1: Allele for D18S535, D1S1656, and D10S2325 STR Loci from Keyboard 1

Keyboard 1
A1 12,12 15,17.3 11,14
A2 12,12 15,17.3 11,14
A3 12,12 15,17.3 11,14
A4 12,12 15,17.3 11,14
B1 12,12 15,17.3 11,14
B2 12,12 15,17.3 11,14
C 12,12 15,17.3 11,14

Table 2: Allele for D18S535, D1S1656, and D10S2325 STR Loci from Keyboard 2

Keyboard 2
A1 9,12,12,14 12,15,17.3 9,11,12,14
A2 9,12,12,14 12,15,17.3 9,11,12,14
A3 9,12,12,14 12,15,17.3 9,11,12,14
A4 9,12,12,14 12,15,17.3 9,11,12,14
B1 12,12 15,17.3 11,14
B2 12,12 15,17.3 11,14
C 12,12 15,17.3 11,14


Test results demonstrated that the authorized user did not contaminate the keyboards with other DNA. Two DNA profiles were found on Keyboard 2 (problem). One profile was compatible with the authorized user’s DNA. The DNA typing results did not identify the second person. A more exhaustive investigation was then initiated, which led to the perpetrator’s identification.


Lareu, M. V., Barral, S., Salas, A., and Carracedo, A. Sequence variation of a variable short tandem repeat at the D18S535 locus, International Journal of Legal Medicine (1998A) 111:337-339.

Lareu, M. V., Barral, S., Salas, A., Pestoni, C., and Carracedo, A. Sequence variation of a hypervariable short tandem repeat at the D1S1656 locus, International Journal of Legal Medicine (1998B) 111:244-247.

Rutty, G. N. An investigation into the transference and survivability of human DNA following simulated manual strangulation with consideration of the problem of third party contamination, International Journal of Legal Medicine (2002) 116:170-173.

Rutty, G. N., Watson, S., and Davison, J. DNA contamination of mortuary instruments and work surfaces: A significant problem in forensic practice? International Journal of Legal Medicine (2000) 114:56-60.

Sweet, D., Lorente, M., Lorente, J. A., Valenzuela, A., and Villanueva, E. An improved method to recover saliva from human skin: The double swab technique, Journal of Forensic Sciences (1997) 42(2):320-322.

Wiegand, P., Lareu, M. V., Schurenkamp, M., Kleiber, M., and Brinkmann, B. D18S535, D1S1656 and D10S2325: Three efficient short tandem repeats for forensic genetics, International Journal of Legal Medicine (1999) 112:360-363.