Silent Witness - Volume 4, Number 5, 1999
A SHORT PRIMER ON STRs
Why Do Prosecutors Need to Learn About STRs?
by Kim Herd, Program Manager, Kimberly Irving, Staff Attorney and Adrianne Day, Legal Intern, APRI's DNA Legal Assistance Unit
Any person who has picked up a newspaper lately is aware that DNA evidence has become an integral part of the American justice system. DNA technology has exploded onto the forensic scene providing evidence that allows juries to convict the guilty and exonerate the innocent. The first DNA technology to be utilized widely in the courtroom involved typing RFLPs (Restriction Fragment Length Polymorphisms). Next, DNA analysts began using a scientific process called Polymerase Chain Reaction (PCR), to conduct DNA forensic testing. PCR-based testing represented a technological advance in that it enabled analysts to type smaller crime scene samples by making many identical copies of the DNA. The next refinement in DNA technology uses the automated analysis of fragments of DNA called Short Tandem Repeats, or “STRs.” In their 1996 Report, the National Research Council stated: "One of the most promising of the newer techniques involves amplification of loci containing STRs…[a]s more STRs are developed and validated, this system is coming into wide use"[1] Indeed, the time has come that STRs are being used by forensic laboratories all over the United States. Prosecutors trying cases involving DNA evidence will likely find that the technology utilized was STRs.
STRs are Faster, Cheaper, and Highly Discriminatory
Even prosecutors who have never presented a case with DNA evidence may still be familiar with the term "STRs." Medical and research laboratories first used STRs in the early 1990's ¾ around the time when PCR technology was moving from the research field into the courtrooms. As early as 1992, a private laboratory contracted with the federal government to employ STRs in the identification of the remains of Desert Storm soldiers. In 1993, officials utilized STR technology to identify individuals killed in the fire that raged through the Branch Davidian Compound outside Waco, Texas. More recently, officials at the Suffolk County Medical Examiner's Office employed STR testing in their efforts to identify victims of the TWA Flight 800 crash off the coast of Long Island, New York. Dr. Charles Wetli of Suffolk County explained that his office employed STRs because of their ability to type badly degraded samples.[2] Most of the remains found after the crash of Flight 800 were small and had been in the water for quite some time.
The ability to retrieve a genetic profile from a severely degraded biological specimen is only one of the many benefits of PCR-based STR profiling. Another such benefit is that STRs lend themselves to automation. After the technician extracts DNA from a biological sample, the DNA is amplified (or copied) using the PCR process. In one method, fluorescent dyes attach at the beginning and end of the target STR sequence, and act as a label for that particular section of DNA. The fluorescently labeled PCR products are copied and then separated via a procedure called gel electropheresis. A laser detects the fluorescently labeled PCR fragments. After a computer assists in the analysis of the genetic profile, the technician then prints out the resulting DNA profile as a graph called an "electropherogram" (see inset). The use of fluorescent STR detection allows multiple DNA samples to be processed more efficiently. Since the process takes less time and requires fewer lab technicians, STR testing results in considerable cost and time savings as compared to older technologies.
STRs are numerous and scattered throughout the humane genome. Although a match at one STR loci is not especially discriminatory, forensic scientists expect that a genetic profile typed from several different STR loci will discriminate between any two individuals (except for identical twins). Given their ability to obtain highly discriminatory DNA profiles from small, degraded samples both faster and less expensively, STRs are fast becoming the DNA testing method of choice of an ever-increasing number of forensic laboratories. However, prosecutors must first understand what STRs are before they can deal with the issues of how to present them in court.
The Science of STR testing
All animals and plants are composed of a collection of specialized cells that have varied roles and functions. Despite their different functions, all human cells (except mature red blood cells) have a nucleus that houses deoxyribonucleic acid (DNA). DNA is the building block of life. Its structure, often described as a twisting ladder, consists of two long strands of sugar and phosphates forming the sides of the ladder, and pairs of nucleotides forming the rungs. DNA nucleotides (called "bases") come in only four types: adenine (A), thymine (T), cytosine (C), and guanine (G). Each strand of DNA contains nearly 3 billion base pairs and would stretch to nearly five feet long if uncoiled.
STRs are small, repeating sequences of DNA, usually three to five base pairs in length. For example, the STR sequence named “THO1” consists of the following four bases: AATG. There are numerous other STR sequences at different “loci” (i.e., locations) scattered throughout the DNA. The THO1 locus, in particular, can repeat anywhere from five to eleven times depending on the person. In other words, one person may have a THO1 profile that looks like: AATGAATGAATGAATGAATG. Whereas, another person may have a THO1 profile of: AATGAATGAATGAATGAATGAATGAATGAATGAATG. It is the variable number of these repeats that distinguishes one profile from another and forms the basis of forensic STR analysis. The combination of profiles derived from several different STR loci can result in a genetic profile that will distinguish one person from another.
STR testing is based on the scientific principles of other generally accepted methodologies. For example, STRs incorporate the concept of repeating sequences found in RFLP and the amplification powers of PCR. However, the STR technique is not simply a newer version of RFLP and PCR. The combination of these techniques results in a unique testing procedure that is "particularly appropriate for forensic use."[3] Therefore, it is not surprising that an ever-increasing number of forensic laboratories are implementing STR systems.
Forensic Laboratories and CODIS are Using STRs
In 1998, the FBI surveyed DNA laboratories and found that 32.1% of casework samples were being analyzed using STR technology.[4] The FBI projected that this percentage would rise to 62.6% in 1999.[5] Of the many laboratories implementing STR technology, several have begun using the specific thirteen core STR loci[6] required by the Combined Offender DNA Index System (CODIS), the FBI’s national DNA database.
CODIS is a DNA database that houses DNA samples of convicted offenders and crime scene samples. CODIS enables state and local forensic laboratories to exchange and compare DNA profiles electronically¾linking serial violent crimes to each other and identifying suspects by matching DNA from crime scenes to convicted offenders or to other crime scenes. CODIS has been a revolutionary tool for law enforcement in that it enables investigators to compare evidentiary samples found at a crime scene with DNA samples collected from known convicted offenders. CODIS has solved many heinous crimes more quickly and efficiently than previously possible, often preventing suspects from re-offending during lengthy investigations and saving precious resources.
The more laboratories that link with CODIS, and the more profiles that are placed in this national database, the more effective CODIS will be at connecting crime scenes and locating suspects. Currently, eleven forensic laboratories are using the thirteen core loci utilized by CODIS.[7] The FBI projected that in 1999, the number of labs using the core loci and linking with CODIS will rise to 123.[8]
Prosecutors need to be aware of this trend among forensic laboratories to move toward STR technology and the 13 core loci adopted by CODIS. If an ever-increasing number of laboratories have STR capabilities, then more and more prosecutors will encounter STRs in their DNA cases. Fortunately, some pioneering prosecutors have already successfully introduced STRs in the courtroom and laid the foundation for STRs to be as readily admissible as their RFLP and PCR forerunners.
STRs are Quickly Gaining Acceptance in Court
Because STRs utilize the accepted technology of PCR typing, a scientific procedure deemed admissible in numerous cases throughout the country, several courts have upheld the admissibility of STR DNA evidence. The Supreme Court of Massachusetts upheld the admission of STR testing in Commonwealth v. Rosier[9] using a Daubert[10] analysis. The admission of the STR testing in this case decreased the probability that the DNA sample obtained from the defendant’s car belonged to someone other than the victim from one in 11,000 to one in 7.5 million. Only a year later in 1998, the Supreme Court of Nebraska in Nebraska v. Jackson[11] admitted STR testing under the traditional Frye[12] model. The Court affirmed that the trial court was correct in determining that the STR test is generally accepted within the scientific community. In fact, appellate courts that have faced this issue have all found STRs to be reliable, scientifically accepted, and relevant tohe prosecutions’ cases.[13]
Conclusion
STRs represent yet another amazing technological advance in the world of DNA forensic evidence. Prosecutors should do what they can to learn about the technology and to take full advantage of its impressive evidentiary potential.
National Research Council, The Evaluation of Forensic DNA Evidence 23, 71 (1996).
Telephone Interview, Dr. Charles Wetli, Suffolk County Medical Examiner's Office (July, 1999).
See supra, note 1, at 117.
FBI Laboratory Forensic Science Systems Unit, 1998 CODIS DNA Laboratory Survey [hereinafter FBI Survey] 12 (1999).
Data for the 1999 CODIS Survey is currently being gathered and is scheduled to be published in late 1999. Telephone Interview, Claire Searby, Federal Bureau of Investigation (Sept. 3, 1999).
The thirteen core loci chosen by the scientific community and CODIS are: D8S1179, D3S1358, FGA, D18S51, D5S818, D13S17, D16S539, CSF1PO, vWA, THO1, D7S820, D21S11, and TPOX.
See FBI Survey at 12.
See FBI Survey at 14.
Commonwealth v. Rosier, 685 N.E.2d 739 (Mass. 1997).
Daubert v. Merrell Dow Pharmaceuticals, 509 U.S. 579 (1993).
Nebraska v. Jackson, 582 N.W.2d 317 (1998).
Frye v. United States, 293 F. 1013 (D.C. Cir. 1923).
See Mississippi v. Wright, 1998 Misc. LEXIS 256; Mississippi v. Watts, 1999 Miss. LEXIS 45; California v. Alton, 1999 Cal. App. LEXIS 566.
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