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Preparing a DNA Case

Jennifer Friedman

Office of the Los Angeles County Public Defender

Updated February 21, 2002




I.                Determine What Physical Evidence Was Tested

a.   The first thing that you will need to do is to determine what evidence was tested and significance of that evidence to the state’s case and to your defense. Examples of biological evidence commonly typed are blood, semen, saliva, and hair. You will not be able to understand the significance of the evidence unless you know what was typed and from where the evidence was collected. You may find it helpful to consult an expert in criminalistics.


b.  At this point you should also determine where the evidence that has been tested is being stored, under what conditions is it being stored and by whom. In addition, is there physical evidence that was collected that has not been tested?


c.   At some point during the case preparation, after you understand the significance of the DNA evidence, you should arrange to inspect all the physical evidence. This includes all the property that was examined at the lab. You should be prepared to photograph the evidence as necessary.


II.            What Evidence Was Not Tested?

a.   You should be aware of evidence that was collected and was not tested. You should also seek to discover if there is physical evidence that was not collected but still may be available. You should consider whether or not to file a motion to preserve the evidence or a motion to require the government to notify you prior to conducting additional testing. A decision to file such a motion is a tactical decision that should be made understanding the significance of the untested evidence to your case.


III.        DNA Typing

a.   In order to evaluate the DNA typing performed at the request of the government you must have a basic understanding of the method of DNA typing employed by the laboratory.

b.  Spend some time reviewing the lab reports. You will also need to familiarize yourself with the basic literature in the field. This will help you begin to learn about the typing method used and the issues associated with the method.

                                                           i.      What type of analysis or typing was done?

1)  It is important to understand what typing method(s) were used in your case. Polymerase Chain Reaction (or PCR) is NOT a method of typing DNA. It is simply a method by which DNA is copied so that there is a sufficient amount of DNA to type or test. Common methods of DNA typing are Restriction Length Polymorphism or RFLP, Reverse dot blot and Short Tandem Repeats or STR’s. It is also important to recognize that there are many different STR systems currently in use. There are significant differences between the different STR systems. Each different method of DNA typing is in a different stage of development with regards to reliability, acceptance, and use in the relevant scientific community.

2)  In addition, scientific opinions regarding such reliability and acceptance change over time. It is incumbent on the lawyer litigating these cases to not only be aware of the relevant case law regarding admissibility of the particular typing method but also be familiar with the latest scientific literature including peer reviewed studies and publications regarding the DNA typing method used in the case. Most of the cases you will see today involve the “short tandem repeat” locations (or loci). However, if you have an older case where the DNA testing was completed years ago, you may see some of these other tests.

a.   You may also see an analysis of Mitochondrial DNA. This is a non-cellular DNA. The type of analysis is commonly conducted on hairs with no root.

3)  There are a number of problems associated with each type of DNA analysis and you will need to become familiar with them. At the bottom of this document is a bibliography of publications and websites that you may refer to for additional information.

                                                        ii.      What lab performed the testing? What kits were used? What hardware was used? What software was used? What database was used?

1)  It is equally important to identify the lab that conducted the testing. Often more than one lab was involved in the testing. There is a great deal of information available about these various labs. You will need to find out everything there is to know about the labs involved in your case.

2)  Many of the labs that conduct DNA typing have had problems over the years, including numerous incidents of:

a.   Contamination

b.  Mislabeling samples

c.   Misrepresentation of information contained in the databases used

d.  Mistakes in proficiency testing

e.  Inadequate validation and failure to gain ASCLD accreditation

3)  At a minimum, you will need to request and/or locate the following information:

a.   The Proficiency tests for the lab technicians who worked on your case

b.  Information pertaining to the accreditation of the lab that performed the testing

c.   Validation studies completed by the lab for the particular testing method used by the lab in your case

d.  Lab error rates and information pertaining to all errors made by the lab since the lab began conducting DNA typing

                                                                                                                                   i.      Names of personnel involved

                                                                                                                                ii.      Type of test performed

                                                                                                                            iii.      Explanation for the error

                                                                                                                             iv.      Changes that were instituted to prevent such problems in the future

                                                                                                                                 v.      The lab’s policy regarding the discipline, etc. of lab personnel who make such errors

e.  A copy of any centralized contamination log delineating instances of contamination in the lab

                                                                                                                                   i.      This will generally be reflected by the contamination showing up in reagent blanks

f.   Information about the company that produces the kits used by the lab (usually either Perkin-Elmer or Promega) and the validation studies done by the company on these kits.

                                                                                                                                   i.      Perkin-Elmer has successfully prevented many defense attorneys from obtaining validation information for the Profiler and Co-filer.

                                                                                                                                ii.      However, in at least three jurisdictions courts have precluded the state from offering STR results as a result.

g.  Information about the company that produces the software and hardware used to analyze the test results. Often the same company will have the supplied the kits, software, and hardware.

h.  You should discover what type of validation was done by the company

                                                                                                                                   i.      Were the validation studies peer-reviewed and published? If not why not?

                                                                                                                                ii.      Was validation done on samples like the samples found in forensic settings?

1.  For example – mixtures, often degraded and contaminated

i.     This is just a sample of the information that you will want to discover in preparing to litigate your case. This is often a time-consuming endeavor. However, if you find that in the process you will often discover things about the lab that are relevant to your case.


IV.         Gathering Information

a.     You should attempt to gather as much information as possible. Here are some ideas for you to consider:

                                                             i.      Proficiency testing and validation done by the individual labs are often available directly from the lab

                                                          ii.      Transcripts of testimony of witnesses who testify both for the state and the defense

                                                       iii.      Articles and studies from scientific journals

1)   For example – “The Journal of Forensic Science”

                                                        iv.      Familiarize yourself with the National Research Council Reports and TWGDAM and/or SWGDAM guidelines

                                                           v.      Case law dealing with admissibility of DNA results from your jurisdiction. You will need to read the out of state opinions and be able to explain to the judge that in the out-of-state case there was no hearing that dealt with the issues you are raising or that record in the out-of-state case is incomplete.

                                                        vi.      You should also purchase at least one primer that explains the essentials

                                                     vii.      Talk to the other lawyers in your office or in other offices around the country who are familiar with these issues. There are a number of lawyers around the country who specialize in this type of litigation.

                                                  viii.      Talk to experts, biologists, geneticists, statisticians, and forensic lab personnel.


V.             Prepare a Discovery Request and Appropriate Subpoenas

a.   Remember that you will never be given everything that you need in order to prepare your case. You will have to make specific requests for the many items. This, of course, requires you to become familiar with the information that exists.

b.  In addition, you may have to go to court to explain to the judge why the information is material. A great deal of the underlying data necessary for the proper evaluation of this evidence will be provided in an electronic format. In order to evaluate the data, you will need to hire an expert who has access to the software that is necessary to read the data.


VI.         Locating and Selecting Experts

a.   Locating an Expert

1)  You should consult with other lawyers in your office and outside your office

2)  Universities are a good source of experts

3)  The Internet can also be a useful tool for locating experts in particular fields.

b.  Selecting an Expert

                                                           i.      This is one of the most important issues you will face in preparing your case for trial. In many, if not all cases, you will need experts in more than one field of expertise. Some individuals feel that it is useful to hire a generalist, that is, an expert who has a background in biology, genetics, statistics, and forensic applications of these areas, who can act as a consultant throughout the preparation phase. Others believe that you should not hire experts until you determine the precise issues in your case.

                                                        ii.      No matter which route you choose to take, you will need to have an understanding of your case before making any decisions about experts.

                                                    iii.      Additionally, you should consult the Forensic Science Group and follow the office procedures prior to hiring an expert.

                                                     iv.      Prior to hiring any expert you should talk to the expert about his or her training and experience.

1)  You should find out if they have testified in other cases.

2)  You should ask them what degrees they hold and where.

3)  You should ask if they have published any articles or studies.

4)  You should spend enough time talking to the expert so that you get a sense about whether you feel comfortable working with this individual and whether the expert communicates well and understands the issues involved in your case.

5)  You should NEVER hire an expert with whom you have not spoken.

                                                         v.      After you hire the expert, you will need to arrange to get all the necessary materials to the expert.

1)  You will also need to meet in person with the expert.

2)  Frequently the experts you hire will live out of state.

a.   You must nevertheless, arrange to meet with your expert(s).

b.  In many situations it may be more cost effective for you to go to your expert.


VII.     Deciding Whether or Not to Re-Test or Conduct Additional DNA Testing

a.   This may be one of the most difficult decisions you will have to make.

                                                           i.      You should be aware of the discovery rules in your jurisdiction and how the government will be able to use the fact that you conducted testing should you choose not to present the results of those tests.

                                                        ii.      Often it makes the most sense to look at the evidence that has not been examined and tested by the government.

1)  You will need to understand what types of substances can be tested as well as the best test for the particular piece of physical evidence you would like tested.

                                                    iii.      In addition you should discuss the issues of what lab should conduct the test.


VIII. Preparing and Litigating a Frye or Daubert Motion

a.   This may be the critical issue in your case.

                                                          i.      The motion you prepare should be well researched and well written.

1) You should attach all the appropriate documents to the motion.

2) There should be an index that makes it easy for the judge to locate particular attachments.

3) You should clearly identify each of the issues.

                                                      ii.      Remember a published decision that purportedly decides the admissibility of a particular method of DNA testing may no longer resolve the issue.

1) The relevant scientific community may have changed or modified its opinion.

2) The issue you wish to litigate may never have been raised in the record of the published case.

a.  The system that was used by the lab in the published case may be different system.


IX.             Preparing Witness to Testify at the Hearing and at Trial

a.     It is often difficult for a layperson with no science in their backgrounds to understand experts.

                                                             i.      You need to spend considerable time with your experts preparing them to testify in such a way as to be understandable to the layperson.

                                                           ii.      Frequently it will be useful to prepare visual aids.

                                                        iii.      Remind the witness not to use scientific terms unless absolutely necessary and then explain what he or she means.

b.     This is a long process and you should be prepared to have more than one preparation session with each of the experts who you will call to testify.


X.                 Decide What Issues You Want to Present to the Jury and Determine the Most Effective Way to Present Those Issues

a.     This may be the most difficult issue to resolve.

                                                             i.      While you may have identified a number of technical problems with the testing performed, or better yet with the interpretation of the results, presenting it to a jury is another matter.

1)    In the limited amount of time available in a trial setting you will need to educate the jury and get them to a level where they understand how these tests work and all of the problems associated with them.

a.     Therefore you will have to:

                                                                                                                                     i.      Choose your issues carefully

                                                                                                                                   ii.      Prepare a visual presentation of those issues

                                                                                                                                iii.      Make sure that you outline them clearly and precisely in your opening and closing arguments.










John Butler, Forensic DNA Typing, Biology and Terminology Behind STR Markers, 2001


NRC, DNA Technology in Forensic Science, 1992


NRC, The Evaluation of Forensic DNA Evidence, 1996


Technical Working Group on DNA Analysis Methods (TWGDAM), Guidelines for Quality Assurance Program for DNA Analysis, 1995


Websites: A Defense Website A Defense Website A list of cases from around the  country in which there have been admissibility challenges to DNA Evidence. This site also has transcripts and/or orders from many of the hearings.


There are numerous forensic science websites. Some of them have general overview of some of the DNA issues.


Carpenter’s Forensic Science Resources –


Reddy’s Forensic Science Home Page –


Zeno’s Forensic Site –


You can find additional websites and articles by using key words “forensic” and “DNA” in the search.


[The Resources page of this site also contains several helpful links]




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