Silent Witness - Volume 5, Number 3, 2000

An Update on STR Admissibility

by Kim Herd, Program Manager and Adrianne Day, Legal Intern,
APRI's DNA Forensics Program

Short Tandem Repeat (STR) PCR-based DNA testing is rapidly becoming the predominant DNA typing technique in U.S. crime laboratories. For years, research and medical laboratories have used commercially available STR kits in procedures ranging from monitoring bone marrow transplant recipients to identifying victims of mass disaster. [See text box for a description of these systems]. In addition, 112 forensic laboratories in 43 states are now connected to the Combined DNA Index System (CODIS), a national DNA database of convicted offender and crime scene profiles.1

Although there are only four appellate decisions affirming the reliability and general acceptance of STR testing,2 the number of trial courts admitting STR evidence is growing.3 Nonetheless, battles over admissibility continue to be waged. Judges in California, Vermont, and Colorado have disallowed DNA evidence derived from various STR systems, including the CTT STR triplex, the Green One kit, Profiler Plus, Cofiler, PowerPlex 1.1, and the ABI 310 or ABI 377 genetic analyzer.4 To successfully use this powerful forensic tool, prosecutors should be aware of defense arguments raised in these cases.

Adherence to TWGDAM guidelines and DAB standards

A key defense argument in the Bokin, Pfenning, and Shreck cases focused on guidelines promulgated by the Technical Working Group on DNA Analysis Methods (TWGDAM). In the early days of forensic DNA, the FBI formed TWGDAM,5 a body of scientists and forensic analysts, to develop quality assurance guidelines for DNA laboratories. TWGDAM recommended procedures for forensic DNA laboratories to follow in such areas as validating new DNA technologies, monitoring analysts' proficiency, and documenting test results. The guidelines took effect in 1991, and TWGDAM updated them in 1995, primarily to address PCR-based systems. Courts evaluating DNA evidence looked to TWGDAM for consensus within the scientific community as to optimal laboratory procedures. Adherence to TWGDAM guidelines was evidence that the laboratory produced reliable results.

The DNA Identification Act of 1994 authorized the creation of the DNA Advisory Board (DAB), an assembly of scientific, legal and ethical professionals, to develop and recommend to the FBI Director quality assurance standards for forensic DNA laboratories. Using the 1995 TWGDAM document as a template, the DAB developed new standards that it submitted to the FBI Director for approval. In October 1998, the FBI Director issued Quality Assurance Standards for Forensic DNA Testing Laboratories for DNA casework laboratories. In April 1999, the FBI Director issued a second set of standards, Quality Assurance Standards for Convicted Offender DNA Databasing Laboratories, for laboratories analyzing convicted offender samples for CODIS. Both sets of standards replaced the TWGDAM guidelines.6 Laboratories intending to enter results into CODIS or those applying for federal grants must comply with these standards (referred to herein as "DAB" standards).

The fact that the DAB standards superseded the TWGDAM guidelines is important because the recommendations vary slightly with respect to two issues defense attorneys are raising in STR admissibility hearings: validation studies and PCR primer sequences. TWGDAM guidelines required that scientists developing new DNA technologies conduct and publish validation studies in peer reviewed scientific journals, and that PCR primer sequences be "readily available to the scientific community."7 The DAB standards do not have these requirements. Nonetheless, defense attorneys have incorrectly argued that laboratories are required to adhere to the more stringent TWGDAM guidelines regarding developmental validation and primer sequences.  

Validation

There are two general categories of validation studies. Manufacturers conduct basic research called, "developmental validation," prior to the release of their products to ensure their reliability, robustness, and accuracy. Sometimes, independent laboratories will also participate in these studies. Laboratories that purchase the kits and utilize them in day-to-day casework perform "internal validation." DAB standards define internal validation studies as "an accumulation of test data within the laboratory to demonstrate that established methods and procedures perform as expected in the laboratory."8

In the three cases that have excluded STR evidence, defense counsel argued that there was insufficient publication of developmental validation studies, rendering the kits non-compliant with TWGDAM, and thus inadmissible. The Pfenning court reasoned that "without the scrutiny of the scientific community, the Court can not establish whether Profiler Plus is a reliable system, or one which is prone to error." (Pfenning, at 50). The court agreed with the defense's contention that failure to follow the TWGDAM guidelines on developmental validation precluded admissibility.

Prosecutors facing this issue should emphasize that the DAB standards and not the TWGDAM guidelines are what forensic DNA laboratories must follow. DAB standards do not require "peer-reviewed" publication of developmental validation studies. Moreover, as one Michigan judge pointed out, "[t]his methodology and the Perkin-Elmer kits as well as the 310 and 377 Genetic Analyzer have been subjected to a staggering amount of what can only be called peer review." (Kopp, at 13). During admissibility hearings, prosecutors may want to submit a list of the myriad scientific articles addressing STRs and their applications in medical and research settings.9

The forensic community is also attempting to address these judicial concerns. Several validation studies are in various stages of publication by the peer-reviewed journal, The Journal of Forensic Sciences. The forthcoming articles deal with both Promega and Applied Biosystems' STR kits. Additionally, The International Journal of Legal Medicine has accepted a European developmental validation study of Profiler Plus.10

Publication of Primer Sequences

Another defense argument has focused on the lack of published "primer sequences" used in the STR kits. Primer sequences are a component of the PCR reaction, and serve two primary purposes. First, primer sequences show the enzymes where to sit down on the sample DNA and begin the PCR process. Second, chemical tags on the primers help the computer differentiate between loci of similar base pair and repeat length. TWGDAM guidelines required that primer sequences be known. The DAB removed this provision from its 1998 standards.

Citing proprietary concerns, manufacturers of commercial STR kits have been reluctant to release their primer sequences. Consequently, defense attorneys in Bokin, Pfenning, and Shreck argued that laboratories using kits with unknown primer sequences were not in compliance with TWGDAM. They argued that knowing the primer sequences is essential for proper validation. In Shreck, the defense expert contended that primers could compete with each other in multiplex systems, inhibiting amplification and causing other problems. (Shreck, at 14).

Accepting the defense's argument, the Pfenning court stated that failure to publish the primer sequences is "anti-scientific in that it inhibits the ability of scientists in the field to test the manufacturers' claims." (Pfenning, at 49). The Shreck court also took a harsh view, stating that the company's "failure to release the primer sequences renders adequate peer review impossible, that there has been no meaningful peer review, and thus there can be no general acceptance by the relevant scientific communities." (Shreck, at 17).

When faced with this argument, prosecutors should emphasize that the current national standards do not require that primer sequences be known. As one judge noted, "Rather than focus on primer sequences it seems that the focus should be on whether the results obtained from kits are reproducible." (Lynch, at 5). He further commented that "[p]rimer sequences are not necessary to see if the kits are working appropriately because labs can validate the results on their own." (Lynch, at 6). Commenting on this same issue in a Minnesota STR admissibility hearing, Judge Thor Anderson stated "The [STR] system is like a Model A. Ford"..[t]housands of owners can tell us it works even if Henry Ford can't or won't explain it." (Dishmon, at 15).

Prosecutors may also argue that, contrary to defense attorneys' contentions, it is not the court's role to resolve such highly technical, scientific issues. Analyzing the validity of each primer sequence for each locus in commercial STR kits is outside the court's expertise. As the Cavin opinion articulates, "[e]xcept perhaps in the rarest of cases, judges cannot accurately resolve competing claims regarding the legitimacy of scientific theories and techniques. Judges can, however, be relied upon to accurately assess whether witnesses have necessary credentials and whether their testimony about how the scientific community feels is persuasive." (Cavin, at 2).

Manufacturers of these kits are addressing courts' concerns regarding primer sequences. Promega is currently publishing an article that includes primer sequence information and has agreed to disclose the sequences prior to publication if necessary. Applied Biosystems will provide their primer sequences under protective order, and have done so in a recent California case.

Conducting a Successful Frye/Daubert Hearing

Far more jurisdictions have admitted evidence from STR kits than have excluded it. Many of those courts considered the same issues as the Bokin, Pfenning, and Shreck courts did. Prosecutors should look to the successes of their colleagues for guidance. In the recent series of Michigan cases admitting STR evidence, prosecutors presented expert witnesses from a wide range of scientific fields who use these STR kits daily. Experts included a medical geneticist, the director of a histocompatibility/immunogenetics lab that monitors the success of bone marrow transplants, and a forensic chemist who conducts drug and explosives analysis. Citing the testimony of these experts, the Kopp court concluded that "by what can only be called overwhelming evidence... the PCR STR methodology as well as the Perkin-Elmer CoFiler and ProFiler kits utilizing the 310 Genetic Analyzer or its relative, the 377 Genetic Analyzer, have received broad and, in fact, universal acceptance in the scientific community." (Kopp, at 21).

In addition to their use by the two largest bone marrow transplant centers in the U.S. and the Human Genome Project, these STR kits were utilized in many of the recent postconviction exonerations. The fact that esteemed medical and scientific professionals use these systems hundreds of thousands of times per day in life-critical procedures should convince the courts that STR kits enjoy general acceptance throughout the scientific community.

Commercial STR Kits

The majority of forensic DNA laboratories use STR kits manufactured by either the Promega Corporation or Applied Biosystems (formerly known as Perkin Elmer). STR kits consist of a multiplex reagent with DNA primers for each of the specific genetic markers and the enzymes necessary for PCR amplification and analysis. Laboratories use semi-automated STR detection instruments and specialized software programs to separate, identify, and interpret DNA profiles from forensic evidence.

PRODUCTS OF PROMEGA

GenePrint^R STR triplex ("CTT") --Types the CSF1PO, TPOX, and THO1 loci using slab gel electrophoresis and silver staining.

GenePrint^R PowerPlex^{TM --} The 1.1 kit types the CTT loci plus the D16S539, D7S820, D13S317, D5S818, and vWA loci. The 2.1 kit has three loci in common with the 1.1 , plus the D8S1179, FGA, D3S1358, D21S11, D18S51, and Penta E loci. Both kits are designed for use with the Hitachi FMBIO^R Fluorescent scanner and the ABI Prism^R 377 DNA Sequencer, resulting in RFLP-like bands.

PRODUCTS OF APPLIED BIOSYSTEMS

AmpFLSTR^R Triplexes --The Green I kit types the same loci as Promega's CTT triplex plus Amelogenin, a sex identifier. The Blue kit types the D3S1358, vWA, and FGA loci. 

AmpFLSTR^R Multiplexes -- The Profiler Plus^TM kit incorporates the Green I and Blue loci plus the D5S818, D13S317, and D7S280 loci. The Cofiler^TM kit types three of the same loci as Profiler Plus^TM plus the D16S539, CSF, THO1 and TPOX loci. Together these kits type the 13 core loci required by CODIS.

All of Applied Biosystems' kits are designed for use with the ABI Prism^R 310 Genetic Analyzer or 377 DNA Sequencer. The 310 uses capillary electrophoresis and fluorescent detection, resulting in colorful peaks along a horizontal baseline.

1 Barry Brown, Address at the 11th International Symposium on Human Identification (Oct. 12, 2000).

2 Mass. v. Rosier, 685 N.E.2d 739 (1997), Neb. v. Jackson, 582 N.W.2d 317 (1998), Watts v. Miss., 733 S.2d 214 (1999), Cal. v. Allen, 72 Cal. App. 4th 1093 (1999).

3 Cal. v. Hunt (Los Angeles County 2000); Mich. v. Cavin, (Lake Co. 2000); Mich. v. Kopp et al., (Kent Co. 2000); Mich. v. Phillips, (Kent Co. 2000); Cal. v. Moevao, (San Francisco Co. 2000); Mo. v. Boyd, (22d D. 2000); Cal. v. Hill, (Santa Barbara Co. 2000); Ariz. v. Lynch, (Maricopa Co. 1999); Mass. v. Gaynor, (Hampden Co. 2000); Utah v. Butterfield, (3d D. 1999); Cal. v. Bertsch and Hronis, (Sacramento Co. 1999); Minn. v. Dishmon, et al., (Hennepin Co. 2000).

4 Cal. v. Bokin, (San Francisco Co. 1999); Vt. v. Pfenning, (Grand Isle Co. 2000); Colo. v. Shreck, (Boulder Co. 2000).

5 In 1999, TWGDAM was changed to “SWGDAM,” for Scientific Working Group on DNA Analysis Methods. Both names are often used interchangeably.

6 Although the DNA Advisory Board will dissolve in December 2000, the DAB standards will remain in effect. SWGDAM will assume the quality assurance/quality control functions of the DAB.

7 TWGDAM, Quality Assurance Standards for Forensic DNA Testing Laboratories, Validation, §§ 4.1.5.12, 4.1.4 (1995).

8 DAB, Quality Assurance Standards for Forensic DNA Testing Laboratories, Definitions, § 2(ff)(2) (1998).

9 See Reference Listing Regarding STRs and DNA Typing (last modified July 24, 2000) <www.cstl.nist.gov/biotech/strbase/STR_ref.htm>.

10 For copies of articles, transcripts, and other materials, please contact the DNA Legal Assistance Unit.