Defense Attorneys


Inform Yourself and Your Client

As a defense attorney, representing a client linked to a crime scene by DNA evidence is a challenge that requires thorough knowledge of rights, science and precedents.

Effective counsel will have an understanding of collection issues, the need for defense experts and possible additional testing, and discovery issues. This module will present these matters.


Case law as it relates to the application of forensic DNA evidence is continually developing. For example, courts have held that DNA evidence is legally sufficient to convict even without identification of the accused by a live witness.

Statutes of Limitation

Jurisdictions have different statutes of limitation that may prevent or hinder the prosecution of suspects identified through DNA databases. Some jurisdictions have either eliminated or extended statutes for specific crimes — such as sexual assault (a crime involved in this case study).

Client Awareness

For a criminal defendant, a DNA evidence case involves many issues.

The client must be informed of the potential for having to submit a blood, saliva or hair sample for testing. DNA may be obtained by court order and refusing to cooperate in a court-ordered DNA test may be admissible as evidence against the defendant.

Counsel should ensure that the client understands how powerful DNA evidence can be. The defendant should understand the consequence of asserting a defense that might contradict DNA analysis results. Likewise, it is important for the defendant to understand that analysis results may support a claim of innocence.

The client should also be informed about how the results of DNA testing were interpreted. It is important to understand the implications of the laboratory's conclusion and its significance in the case.

The defense should let the client know that additional testing and expert support may be requested. The defendant should be made aware that additional DNA testing might result in evidence that may be detrimental to the case. More information on expert support and testing is presented later in this module.

Chain of Custody

In the case study, many items with biological material that were collected as evidence and tested at a laboratory may be presented in court proceedings as evidence against the accused. The defense should consider the following:

  • Was the evidence properly collected, packaged, and stored?
  • Was the chain of custody maintained and documented?


In a case involving DNA evidence, it is essential that the defense obtain complete discovery as early as possible. This may be achieved through discovery requests and/or motions.

Discovery rules often provide for defense access to physical evidence on request or by motion to the trial court. Discovery rules generally mandate government disclosure of reports of examination and tests if the prosecution intends to use those results or if they are material to preparing the defense.

Discovery Requests: Testing

It is important to identify the laboratory that was used and the examiners who conducted the analysis (as well as their qualifications). It should be determined which tests were conducted and on what items. The following should be obtained:

  • Laboratory reports
  • Underlying test data and bench notes
  • Laboratory chain-of-custody documentation

Discovery Request: Quality Assurance

Quality assurance is a critical component in assessing the validity and weight of test results.

A discovery request may seek information about the laboratory's:

  • Quality assurance program
  • Quality control documentation

To obtain the most accurate information regarding the quality of the analysis performed by the laboratory, the discovery request should also seek to obtain:

  • Proficiency test results for DNA scientists
  • Information concerning laboratory protocols and procedures, incidences of contamination results, maintenance and calibration, accreditation status and proficiency tests
  • Standard operating procedures including the quality assurance manual, calibration records for laboratory instrumentation used in the case and the proficiency testing record of the scientist

Responding to Discovery Requests

By statute or rule, the defense may be required to make pretrial disclosure to the prosecution, especially when it intends to present expert testimony. Failure to comply could result in sanctions up to and including exclusion of the expert's evidence. Reference should be made to the jurisdictional law.

Exculpatory Evidence

While exculpatory evidence is material in criminal cases, it is particularly significant when dealing with biological evidence. The defense must be aware of both the prosecutor's duty to disclose exculpatory evidence and the jurisdictional requirements for the preservation of evidence. In some cases, there may be an opportunity, statutory or otherwise, to seek pretrial DNA testing.

New Testing

The defense need not, and should not, test every item from the crime scene without full consultation with the accused and the defense expert, as some tests may be irrelevant or may provide evidence damaging to the defense.

Obtaining New Reference Samples

The defense should be aware that it may be necessary for reference samples (DNA from people who had access to the crime scene) to be collected and analyzed for comparison to crime scene evidence. Reference samples can be obtained by consent or by court order.

Obtaining the Suspect's Sample

DNA, like a blood-alcohol sample, is nontestimonial evidence. Thus, there is no violation of the Fifth Amendment privilege against compelled self-incrimination when DNA evidence is lawfully seized.

A Motion to Secure a Defense Expert

The evaluation of DNA test results may require expert assistance. An expert can educate and advise the defense team and possibly serve as a witness. If the defendant is indigent, an expert can be obtained by motion to the court requesting funding for such assistance.

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Paying for Expert Assistance

The defendant in the case study is indigent. He has a constitutional right to expert assistance financed by the court or prosecuting county. This right is not absolute; there will be an assessment of the probable value of the assistance sought and the risk of error if such assistance is not obtained. Thus, the defense motion needs to detail the client's indigent status and the specific reason(s) the expert assistance is required.

The Roles and Benefits of an Expert

An expert for the defense can help with complex and technical matters, such as:

  • Interpreting the prosecution's laboratory report and underlying data
  • Determining whether there were problems with the laboratory analysis and/or evidence handling
  • Evaluating the need for independent testing and observing additional testing procedures
  • Offering testimony at trial
  • Preparing counsel for the presentation of DNA evidence at trial
  • Determining whether to challenge the admissibility of newer DNA technologies

The importance of a DNA expert's help should not be understated. The expert can determine whether degradation or contamination has affected the results. The expert can also assist with evaluating mixtures or partial profiles and statistical statements or conclusions reported by the laboratory.

The expert can interpret the technical language contained in laboratory reports.

For example, DNA analysis results are reported differently by different laboratories. Some laboratories report results as an "inclusion," but others use the term "match." An inclusion indicates that a person cannot be excluded. A match, however, is much stronger language. Some laboratories do not use either term and simply report that crime scene evidence profiles are "consistent with" a suspect profile.

Helping to Counsel Your Client

An expert can help with client communication by ensuring that the defendant is sufficiently informed to make critical decisions.

For example, an expert may help the accused understand the weight and significance of the government's evidence prior to trial. This knowledge facilitates an informed decision to either go to a trial or enter a plea.

Motion in Limine

A motion in limine is a request to bar, limit or, in some cases, admit specific pieces of evidence.

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The defense in the case study will move in limine to preclude mention of the fact that the accused was arrested after the DNA from the crime scene was found to match his DNA profile in the Combined DNA Index System (CODIS), a criminal DNA database.

Ex Parte Motion

A motion to obtain funds for expert services may reveal a theory of defense. Thus, such motions are often filed ex parte and heard in camera with the record from the hearing sealed. This process protects defense strategies while permitting appellate review if the request is denied.

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Motion to Suppress

A motion to suppress seeks to exclude evidence seized in violation of constitutional (or sometimes statutory) rights. Challenges can be brought when the defendant's DNA was obtained through an unconstitutional search and seizure.

Issues may include the validity and scope of consent, the lawfulness of a stop or seizure, and whether a warrant was required.

The Defense Attorney's Role

Although challenges to DNA evidence are becoming less frequent, it is the responsibility of the defense attorney to investigate and present challenges that are relevant to a client's defense. Defense challenges to DNA evidence generally revolve around three major themes: contamination, testing irregularities, and statistical analysis.

DNA-related issues will be critical at each of several stages:

  • Voir dire
  • Opening statement
  • Examination of the evidence collection witness(es)
  • Examination of the DNA expert witness(es)
  • Closing argument

Expert Witness Qualification

For the government DNA expert witness(es), the defense will have to determine whether a challenge to credentials will succeed at the voir dire of the witness when he or she is being proffered by the prosecution as an expert. If the witness's credentials are strong, the defense may elect to stipulate to expertise.

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Should the government witness be qualified by the court, he or she might be able to validate portions of the defense theory. In the case study, for example, the government expert confirms that an 11-loci correspondence does not exclude all other suspects. Additionally, the expert validates the fact that DNA analysis methods cannot determine when biological material was deposited. If a witness is to be impeached, it is important that it is executed properly and that it is built around points such as bias, inconsistent statements, and/or competency.