Title: Understanding DNA Evidence: A Guide for Victim Service 
Providers
Series: Brochure
Author: National Institute of Justice and Office for Victims of Crime
Published: National Institute of Justice, May 2001
Subject: Technology in law enforcement, evidence, rape and sexual
assault, and victim services
8 pages
14,000 bytes

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Understanding DNA Evidence: A Guide for Victim Service Providers

DNA evidence is playing a larger role than ever before in criminal cases 
throughout the country, both to convict the guilty and to exonerate those 
wrongly accused or convicted. This increased role places greater 
importance on the ability of victim service providers to understand the 
potential significance of DNA evidence in their clients' cases.

What Is DNA? 

DNA, or deoxyribonucleic acid, is the building block for the human body; 
virtually every cell contains DNA. The DNA in people's blood is the same 
as the DNA in their saliva, skin tissue, hair, and bone. Importantly, DNA 
does not change throughout a person's life.

The Value of DNA Evidence 

DNA is a powerful investigative tool because, with the exception of 
identical twins, no two people have the same DNA. Therefore, DNA 
evidence collected from a crime scene can be linked to a suspect or can 
eliminate a suspect from suspicion. During a sexual assault, for example, 
biological evidence such as hair, skin cells, semen, or blood can be left on 
the victim's body or other parts of the crime scene. Properly collected 
DNA can be compared with known samples to place a suspect at the scene 
of the crime. In addition, if no suspect exists, a DNA profile from crime 
scene evidence can be entered into the FBI's Combined DNA Index 
System (CODIS) to identify a suspect anywhere in the United States or to 
link serial crimes to each other. 

The effective use of DNA as evidence may also require the collection and 
analysis of elimination samples to determine the exact source of the DNA. 
Elimination samples may be taken from anyone who had lawful access to 
the crime scene and may have left biological material. When investigating 
a rape case, for example, it may be necessary to obtain an elimination 
sample from everyone who had consensual intercourse with the victim 
within 72 hours of the alleged assault to account for all of the DNA found 
on the victim or at the crime scene. Comparing DNA profiles from the 
evidence with elimination samples may help clarify the results.

Evidence Collection 

Victim service providers, crime scene technicians, nurse examiners, and 
other medical personnel should be aware of important issues involved in 
identifying, collecting, transporting, and storing DNA evidence. If DNA 
evidence is not initially identified at the crime scene or on the victim, it 
may not be collected, or it may become contaminated or degraded.

To assist in collection, victims of sexual assault should not change clothes, 
shower, or wash any part of their body after the assault. Such evidence as 
semen, saliva, and skin cells may be found on clothing or bedding, under 
fingernails, or in the vaginal, anal, or mouth region.

Evidence on or inside a victim's body should be collected by a physician 
or sexual assault nurse examiner. A medical examination should be 
conducted immediately after the assault to treat any injuries, test for 
sexually transmitted diseases, and collect forensic evidence, such as 
fingernail scrapings and hair. Typically, the vaginal cavity, mouth, anus, 
or other parts of the body that may have come into contact with the 
assailant are examined.

The examiner should also take a reference sample of blood or saliva from 
the victim to serve as a control standard. Reference samples of the victim's 
head and pubic hair may also be collected if hair analysis is required. A 
control standard is used to compare known DNA from the victim with that 
of other DNA evidence found at the crime scene to determine possible 
suspect(s).

Given the sensitive nature of DNA evidence, victim service providers 
should always contact crime laboratory personnel or evidence technicians 
when procedural collection questions arise.

Contamination and Preservation Issues 

DNA evidence can become contaminated when DNA from another source
gets mixed with DNA relevant to the case. For this reason, investigators 
and laboratory personnel should always wear disposable gloves, use clean 
instruments, and avoid touching other objects, including their own body, 
when handling evidence.

Environmental factors, such as heat and humidity, can also accelerate the 
degradation of DNA. For example, wet or moist evidence that is packaged 
in plastic will provide a growth environment for bacteria that can destroy 
DNA evidence.

Therefore, biological evidence should be thoroughly air dried, packaged in 
paper, and properly labeled. Handled in this manner, DNA can be stored 
for years without risk of extensive degradation, even at room temperature. 
For long-term storage issues, contact the local crime laboratory.

DNA Testing 

The most common form of DNA analysis is called polymerase chain 
reaction (PCR). PCR has allowed investigators to successfully analyze 
evidence samples of limited quality and quantity. The PCR process makes 
millions of copies of very small amounts of DNA. This enables the 
laboratory to generate a DNA profile, which can be compared with the 
DNA profile from a suspect. A statistic is then generated to reflect how 
often one would expect to find this particular DNA profile in the general 
population.

Interpreting DNA Test Results 

Three types of results can occur in DNA testing: inclusion, exclusion, and 
inconclusive. It is important that victim service providers understand the 
meaning of these terms and be able to explain their implications.

Inclusion. When the DNA profile of a victim or suspect is consistent with 
the DNA profile from the crime scene evidence, the individual is 
"included" as the possible source of that evidence. However, the strength 
of inclusion depends upon the number of loci (locations on the DNA 
strand) examined and how common or rare the resulting DNA profile is in 
the general population.

Exclusion. When the DNA profile from a victim or suspect is inconsistent 
with the DNA profile generated from the crime scene evidence, the 
individual is "excluded" as the donor of the evidence. However, exclusion 
does not imply innocence. In a rape case, for example, a perpetrator 
wearing a condom could be excluded as a suspect because no semen was 
found at the crime scene, but evidence found elsewhere at the crime scene
may include that same person as a suspect.

Inconclusive. Inconclusive results indicate that DNA testing could neither 
include nor exclude an individual as the source of biological evidence. 
Inconclusive results can occur for many reasons: for example, the quality 
or quantity of DNA may be insufficient to produce interpretable results, or 
the evidentiary sample may contain a mixture of DNA from several 
individuals (e.g., a sample taken from a victim of a gang rape).

As with all DNA testing results, additional testing may be needed and 
findings should be interpreted in the context of other evidence in the case.

DNA Evidence in Unsolved and Postconviction Cases 

Advanced DNA technology, such as PCR, makes it possible to obtain 
conclusive results in cases in which previous testing might have been 
inconclusive. This can result in the identification of suspects in previously 
unsolvable cases or the exoneration of those wrongfully convicted. It is 
important to realize that while testing or retesting may exonerate an 
individual, exclusionary results may not necessarily prove actual 
innocence. Prosecutors, defense counsel, the court, and law enforcement 
should confer on the need for testing on a case-by-case basis.

Using CODIS To Solve Crime 

CODIS uses two indexes to generate investigative leads in crimes that 
contain biological evidence. The forensic index contains DNA profiles 
from biological evidence left at crime scenes, and the offender index 
contains DNA profiles of individuals convicted of violent crime. Each 
State in the Nation has a DNA database law that defines which convicted 
offenders must have their profiles entered into CODIS; some States even 
require that DNA profiles from all felons be entered into the database. 
CODIS enables Federal, State, and local forensic crime laboratories to 
work together--between jurisdictions and across State lines--to solve 
crimes.

Identifying DNA Evidence 

Since only a few cells are needed for a useful DNA sample, the list below 
identifies some areas at the crime scene or on the victim that may contain 
valuable DNA evidence. Remember, even though a stain cannot be seen, 
there may be enough cells for DNA typing. Furthermore, DNA does more 
than just identify the source of the sample; it can place a known individual 
at a crime scene, in a home, or in a room where the suspect claimed not to 
have been. The more victim service providers know about properly 
identifying, collecting, and preserving DNA evidence, the more powerful a 
tool it becomes.

Possible Location of DNA Evidence--Source of DNA

o Bite mark or area licked--Saliva
o Fingernail scrapings--Blood or skin cells
o Inside or outside surface of used condom--Semen or skin cells
o Blankets, sheets, pillows, or other bed linens--Semen, sweat, hair, or
saliva 
o Clothing, including under garments worn during and after the assault--
Hair, semen, blood, or sweat 
o Hat, bandanna, or mask--Sweat, skin cells, hair, or saliva
o Tissue, washcloth, or similar item--Saliva, semen, hair, skin cells, or
blood
o Cigarette butt; toothpick; or rim of bottle, can, or glass--Saliva
o Dental floss--Semen, skin cells, or saliva
o Tape or ligature--Skin cells, saliva, or hair

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Suggested Resources

Agencies and organizations

American Prosecutors Research Institute
99 Canal Center Plaza, Suite 510
Alexandria, VA 22314
703-549-4253
www.ndaa-apri.org/apri/Index.html

Federal Bureau of Investigation
J. Edgar Hoover Building
935 Pennsylvania Avenue N.W.
Washington, DC 20535
202-324-3000
www.fbi.gov

National Center for Victims of Crime
2111 Wilson Boulevard, Suite 300
Arlington, VA 22201
703-276-2880
www.ncvc.org

National Commission on the Future of DNA Evidence
Office of Justice Programs
National Institute of Justice
810 Seventh Street N.W.
Seventh Floor
Washington, DC 20531
202-307-0645 
www.ojp.usdoj.gov/nij/dna

National Criminal Justice Reference Service (NCJRS)
P.O. Box 6000
Rockville, MD 20849-6000
800-851-3420
www.ncjrs.org

Office for Victims of Crime
U.S. Department of Justice
810 Seventh Street N.W.
Washington, DC 20531
202-307-5983
www.ojp.usdoj.gov/ovc

Office for Victims of Crime Resource Center
P.O. Box 6000
Rockville, MD 20849-6000
800-627-6872 
www.ojp.usdoj.gov/ovc/ovcres

Rape, Abuse & Incest National Network
635B Pennsylvania Avenue S.E.
Washington, DC 20003
800-656-HOPE
www.rainn.org

Sexual Assault Nurse Examiner (S.A.N.E.)-Sexual Assault Response 
Team (S.A.R.T.)
www.sane-sart.com

Speaking Out About Rape Inc. (S.O.A.R.)
69 East Pine Street
Orlando, FL 32801
407-836-9692
www.soar99.org

Publications

Connors, Edward, Thomas Lundregan, Neal Miller, and Tom McEwen. 
Convicted by Juries, Exonerated by Science: Case Studies in the Use of 
DNA Evidence To Establish Innocence After Trial. Research Report. NCJ 
161258. Washington, DC: U.S. Department of Justice, National Institute 
of Justice, 1996.

Hammond, Holly A., and C. Thomas Caskey. Automated DNA Typing: 
Method of the Future? Research Preview. NCJ 184373. Washington, DC: 
U.S. Department of Justice, National Institute of Justice, 1997.

Handbook of Forensic Services: Evidence Examinations. Washington, 
DC: U.S. Department of Justice, FBI, 1999.

Inman, Keith, and Norah Rudin. An Introduction to Forensic DNA 
Analysis. Boca Raton, FL: CRC Press Inc., 1997.

Ledray, Linda E. Sexual Assault Nurse Examiner (S.A.N.E.) Development 
and Operation Guide. NCJ 170609. Washington, DC: U.S. Department of 
Justice, Office for Victims of Crime, 1999. 

McKenna, Judith A., Joe S. Cecil, and Pamela Coukos, Jr. "A Reference 
Guide on Forensic DNA Evidence." In Reference Manual on Scientific 
Evidence. Washington, DC: Federal Judicial Center, 1994.

National Commission on the Future of DNA Evidence. Postconviction 
DNA Testing: Recommendations for Handling Requests. NCJ 177626. 
Washington, DC: U.S. Department of Justice, National Institute of Justice, 
1999.

National Commission on the Future of DNA Evidence. What Every Law 
Enforcement Officer Should Know About DNA Evidence. BC 000614.
Washington, DC: U.S. Department of Justice, National Institute of Justice, 
1999.

National Research Council. DNA Technology in Forensic Science. 
Washington, DC: National Academy Press, 1992.

National Research Council. The Evaluation of Forensic DNA Evidence. 
Washington, DC: National Academy Press, 1996.

Weedn, Victor Walter, and John W. Hicks. The Unrealized Potential of 
DNA Testing. Research in Action. NCJ 170596. Washington, DC: U.S. 
Department of Justice, National Institute of Justice, 1998.

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BC 000657