So good lawyers shouldn’t let the DNA evidence become the case. It’s just a tool, like a fingerprint. Nothing more. It’s not magic, it’s not infallible. There’s plenty of room for error.”

                                                                                     - Attorney Nathanial Burney

“Two tacks:

  1. Challenging the analysis, e.g.:

             • Contamination

             • Interpretation error

             • Mislabeling or other clerical errors

             • Fraud

     2.  Accepting the analysis, but minimizing its significance

Remember—DNA alone cannot prove guilt (or innocence).  It only matters in the context of the rest of the evidence.”

                                                                                                          - Attorney Keith Findley


From the Champion, April 2003:

Two types of DNA testing

  1.  STR – examines short tandem repeats - most common
  2. examines mitochondrial DNA 

First, get the DNA lab report.  This is a summary that the lab routinely provides to the prosecutor’s office.

 The report should state what samples were tested, what type of DNA test was performed, and which samples could (and could not) have a common source. Reports generally also provide a "table of alleles" showing the DNA profile of each sample. The DNA profile is a list of the alleles (genetic markers) found at a number of loci (plural for "locus," a position) within the human genome. To understand DNA evidence, you must first understand the table of alleles.

When analyzing DNA, scientists ideally focus on thirteen markers, known as loci. The odds of finding two people who share all thirteen is roughly on par with those of being hit by an asteroid—about one in a quadrillion in many cases. But the fewer the markers, the higher the probability that more than one person will match the same profile, since relatives often share a number of markers and even perfect strangers usually share two or three. 

From DNA for the Defense:

What a lab report generally contains:

■■ Administrative information on the case, such as agency, case identifier (e.g., file number),

evidence item numbers, victim name and suspect name.

■■ Date the report was issued and a signature and title, or equivalent identification, of the

person accepting responsibility for the content of the report.

■■ A description of the evidence examined.

■■ A description of the technology or technique used.

■■ Locus or amplification system used.

■■ Testing results and/or conclusions.

■■ A quantitative or qualitative interpretive statement.

■■ Disposition of evidence.




Possible­DNA­report­conclusions (from DNA for the Defense)

  1.  an inclusion (DNA match)

An inclusion, or DNA match, occurs when a known sample is compared with an evidence

sample and the profiles are the same. An inclusion or a DNA match may also occur when all of

the alleles in a known sample are also found in a DNA mixture profile. The significance of the

inclusion or match will depend on the statistical data obtained. Some labs report this finding as

“cannot be excluded.”

  1.  an exclusion (DNA non-match)

When a known sample is compared with an evidence sample, the donor of the known sample is

excluded as a source of the evidence if the profiles are different. This is referred to as an exclusion (or a DNA nonmatch).

When an individual is excluded as the potential source of DNA, it does not necessarily mean

the individual was not involved. For example, a true perpetrator who left no detectable biological material will be excluded as a source of DNA.

Conversely, if an individual is a potential source of DNA at a crime scene, it does not necessarily

mean that the person was involved in the crime.

  1. No conclusion

 Sometimes, no conclusion can be drawn as to whether a known individual is included or excluded as the potential source of DNA evidence.

Inconclusive or uninterpretable results may be due to complicating factors such as multiple contributors, contamination, degradation of samples, or misinterpretation or misrepresentation of the results.

  1. No results

Sometimes testing of a sample is attempted, but no results are obtained. This could indicate:

■■ Absence of DNA in the sample.

■■ Insufficient DNA in the sample.

■■ Extensively degraded DNA.

■■ Presence of a substance that inhibits the PCR process (PCR inhibitor), such as denim dyes,

carpet glue or certain types of latent print powder.

■■ Improperly conducted or incomplete testing.



Resulting DNA profile

  1.  Single-source­profile

DNA from one contributor is commonly referred to as a single-source DNA profile. A single-source profile could be derived from:

■■ A reference sample (victim or suspect).

■■ An elimination sample (first responders, EMT personnel, consensual sex partners, or anyone

who might have had authorized access to the crime scene).

■■ A crime scene or other evidence sample (blood stain, chewed gum, cigarette butt).

  1.  More­than­one­source

Mixtures of DNA from more than one contributor are commonly encountered. A mixture could be due to:

■■ Actual contribution by multiple donors during the crime.

■■ Presence of DNA on the item prior to the crime.

■■ Testing of intimate samples (e.g., vaginal swabs or breast swabs).

■■ DNA added by handling an item after a crime but before recovery (e.g., handling of a gun

used in a crime by a person(s) other than the police).

■■ Contamination during crime-scene processing and sample handling (collection, packaging or

testing). Any biological material (blood, semen, saliva, urine, hair, sweat and skin cells left behind after contact) can be mixed and found in combination with any other biological material.






Discovery:  More than lab reports, get entire DNA case file from lab.

Examine the underlying lab data to see if the report conclusions are supported by the data.  This requires a specific demand for discovery to the lab because it is not typically provided to the prosecutor – but is another one of those things that really should be provided.

Get the lab analyst’s bench notes and raw data.  Examine the entire crime lab file.

Look for indicators of bias

Look for indicators of errors (contamination in controls, minor DNA peaks that are disregarded, bad buffers or chemicals, etc.)

Case files commonly include:

■■ A chain of custody for items received by the laboratory.

■■ Sketches or photographs taken in the laboratory.

■■ Examination (“bench”) notes of any testing by the analyst.

■■ Laboratory logs or standard forms related to testing.

■■ Strips, photographs or copies of autoradiographic film or electropherogram data.

■■ Communication between the analyst and others involved in the case.

In addition to information in the case file, other lab documents may include:

■■ Equipment calibration and maintenance records.

■■ Analyst training and proficiency test records.

■■ Unexpected results or corrective action reports.

■■ Quality assurance reports and audits

Behind the Table of Alleles Detected is a set of computer-generated graphs called electropherograms that display the test results. When evaluating STR evidence, a defense lawyer should always examine the electropherograms because they sometimes reveal unreported ambiguities and, fairly frequently, evidence of additional, unknown contributors. 

A number of factors can introduce ambiguity into STR evidence, leaving the results open to alternative interpretations. To competently represent an individual incriminated by DNA evidence, defense counsel must uncover these ambiguities, when they exist, understand their implications, and explain them to the trier-of-fact. 

The best experts for evaluating whether the lab’s finding are supported by the underlying data are academic scientists in the fields of molecular biology, biochemistry, bioinformatics, molecular evolution, genetics (particularly human and population genetics), and related fields. It is not essential that the expert have had experience analyzing forensic samples. In fact, we find that forensic scientists often (but not always) make poor defense experts because they tend to accept too readily the goal-directed subjective judgments and circular reasoning of their crime lab colleagues.

Academic scientists generally have much stronger training in scientific methods and, as a result, demand that test results be interpreted in a scientifically rigorous and unbiased manner.  They often are appalled at the willingness of some forensic scientists to rely on subjective judgment and guesswork to resolve ambiguities in scientific data and their unwillingness to utilize blind procedures when making such judgments.

Problems with DNA interpretation


One of the most common complications in the analysis of DNA evidence is the presence of DNA from multiple sources. A sample that contains DNA from two or more individuals is referred to as a mixture. A single person is expected to contribute at most two alleles for each locus. If more than two peaks are visible at any locus, there is strong reason to believe that the sample is a mixture. 

By their very nature mixtures are difficult to interpret. The number of contributors is often unclear. Although the presence of three or more alleles at any locus signals the presence of more than one contributor, it often is difficult to tell whether the sample originated from two, three, or even more individuals because the various contributors may share many alleles

Some laboratories try to determine which alleles go with which contributor based on peak heights. They assume that the taller peaks (which generally indicate larger quantities of DNA at the start of the analysis) are associated with a "primary" contributor and the shorter peaks with a "secondary" contributor.. . . But these inferences are often problematic because a variety of factors, other than the quantity of DNA present, can affect peak height. Moreover, labs are often inconsistent in the way they make such inferences, treating peak heights as a reliable indicator of DNA quantity when doing so supports the government's case, and treating them as unreliable when it does not. 

These interpretive ambiguities make it difficult, and sometimes impossible, to estimate the statistical likelihood that a randomly chosen individual will be "included" (or, could not be "excluded") as a possible contributor to a mixed sample. Defense lawyers should look carefully at the way in which laboratories compute statistical estimates in mixture cases because these estimates often are based on debatable assumptions that are unfavorable to the defendant. 

When analyzing DNA, scientists ideally focus on thirteen markers, known as loci. The odds of finding two people who share all thirteen is roughly on par with those of being hit by an asteroid—about one in a quadrillion in many cases. But the fewer the markers, the higher the probability that more than one person will match the same profile, since relatives often share a number of markers and even perfect strangers usually share two or three. 


As samples age, DNA like any chemical begins to break down (or degrade). This process occurs slowly if the samples are carefully preserved but can occur rapidly when the samples are exposed for even a short time to unfavorable conditions, such as warmth, moisture or sunlight. 

Degradation skews the relationship between peak heights and the quantity of DNA present. Generally, degradation produces a downward slope across the electropherograms in the height of peaks because degradation is more likely to interfere with the detection of longer sequences of repeated DNA (the alleles on the right side of the electropherogram) than shorter sequences (alleles on the left side). 

Degraded samples can be difficult to type. The process of degradation can reduce the height of some peaks, making them too low to be distinguished reliably from background "noise" in the data, or making them disappear entirely, while other peaks from the same sample can still be scored. In mixed samples, it may be impossible to determine whether the alleles of one or more contributors have become undetectable at some loci. Often analysts simply guess whether all alleles have been detected or not, which renders their conclusions speculative and leaves the results open to a variety of alternative interpretations. Further, the two or more biological samples that make up a mixture may show different levels of degradation, perhaps due to their having been deposited at different times or due to differences in the protection offered by different cell types. Such possibilities make the interpretation of degraded mixed samples particularly prone to subjective (unscientific) interpretation. 

Allelic Dropout.

In some instances, an STR test will detect only one of the two alleles from a particular contributor at a particular locus. Generally this occurs when the quantity of DNA is relatively low, either because the sample is limited or because the DNA it contains is degraded, and hence the test is near its threshold of sensitivity. The potential for allelic dropout complicates the process of interpretation because analysts must decide whether a mismatch between two profiles reflects a true genetic difference or simply the failure of the test to detect all of the alleles in one of the samples. 

Spurious Peaks.

An additional complication in STR interpretation is that electropherograms often exhibit spurious peaks that do not indicate the presence of DNA. These extra peaks are referred to as "technical artifacts" and are produced by unavoidable imperfections of the DNA analysis process. The most common artifacts are stutter, noise and pull-up. 

Stutter peaks are small peaks that occur immediately before (and, less frequently, after) a real peak. Stutter occurs as a by-product of the process used to amplify DNA from evidence samples. In samples known to be from a single source, stutter is identifiable by its size and position. However, it is sometimes difficult to distinguish stutter bands from a secondary contributor in samples that contain (or might contain) DNA from more than one person. 

"Noise" is the term used to describe small background peaks that occur along the baseline in all samples. A wide variety of factors (including air bubbles, urea crystals, and sample contamination) can create small random flashes that occasionally may be large enough to be confused with an actual peak or to mask actual peaks. 

Pull-up (sometimes referred to as bleed-through) represents a failure of the analysis software to discriminate between the different dye colors used during the generation of the test results. A signal from a locus labeled with blue dye, for example, might mistakenly be interpreted as a yellow or green signal, thereby creating false peaks at the yellow or green loci. Pull-up can usually be identified through careful analysis of the position of peaks across the color spectrum, but there is a danger that pull-up will go unrecognized, particularly when the result it produces is consistent with what the analyst expected or wanted to find. 

Although many technical artifacts are clearly identifiable, standards for determining whether a peak is a true peak or a technical artifact are often rather subjective, leaving room for disagreement among experts. Furthermore, analysts often appear inconsistent across cases in how they apply interpretive standards — accepting that a signal is a "true peak" more readily when it is consistent with the expected result than when it is not. Hence, these interpretations need to be examined carefully.




Educate Yourself

• Obtain the accreditation manual for the Association of Crime Laboratory Directors/Laboratory Accreditation Board (ASCLD/LAB)

• Obtain the NAS Report on Forensic Sciences (2009) (note that the NRC has also published

two Reports on DNA analysis (1992 & 1996)

• Obtain Reports of FBI’s Scientific Working Group on DNA Analysis Methods (SWGDAM)


Consult an Expert

– Have expert analyze the raw data to identify possible sources of error

– Have the expert identify uncertainties or questions that can be raised in cross-examination

– Have the expert identify assumptions underlying analysis of mixed samples

– Have the expert identify additional or alternative DNA tests that might be productive



Challenges to Reliability

• Contamination

– As DNA testing becomes increasingly sensitive, it becomes more susceptible to detecting minute contaminants

– Especially problematic with highly sensitive techniques such as single cell PCR and mtDNA



What should be requested from the lab?

1. A disk containing raw data, including but not limited to the sample files, project files,

injection lists, sample sheets and injection logs.


Start by reviewing the injection lists, sample sheets and injection logs, noting the time and

date stamps on all runs to check the order in which samples were run and to make sure that

controls were not substituted with those from a different day. This could reveal a mistake or, in

the most extreme circumstance, indicate laboratory fraud; in either case, conclusions based on

the evidence and reference profiles would be unreliable. If it was an honest mistake, the lab

should be able to provide data for the actual controls run on that day. If controls did not perform

properly or were not run, conclusions based on the evidence and reference profiles would be

considered invalid.


2. Copies of the DNA typing results.

The electropherograms are frequently printed out by laboratory analysts. These printouts may

be color copies, which are ideal, or they may be black-and-white photocopies, which are somewhat harder to read. Regardless, the printouts are often easier to view than the electronic files because they do not require a computer or a license to run the computer programs that generate the electropherograms. In fact, many DNA analysts make allele determinations based on the printouts, not the electronic files. Analysts will often write notes on these printouts, such

as notations on peaks that were considered artifacts, descriptions of the baseline, and calculations of peak height ratios to distinguish stutter from real allele peaks and distinguish heterozygous peaks from peaks of different contributors. These notes are also likely to be initialed by

the analyst making the calls. The defense should consider whether an abundance of artifacts possibly indicates unreliable data or artifacts that are masking true alleles. Defense counsel should obtain copies of all printouts retained by the laboratory.

3. Copies of real-time, slot blot or other quantitation data.

Knowledge about the quantity of DNA obtained is very important and is acquired using different

methods, the most common current method being real-time PCR, also called qPCR. Each

quantitation method contains calculations for estimating how much DNA was extracted from

each sample (some labs also determine how much human male DNA is present). This information allows the defense to determine how much human DNA was used and how much remains from each sample, which could be retested



[continue discovery from lab]

[other discovery issues]

[how does results fit into theory of defense]

[expert or not]

[motions in limine]

[other pretrial issues]

[dna at trial – jury selection, opening, admissibility of evidence, closing, jury instructions]




Appendix: Model Discovery Request for STR Test Results




This is a request for disclosure of scientific materials pertaining to DNA testing performed in the case of [case name] ([County, Case Number]). This request applies to all

DNA testing that has been, is currently being, or will be performed in the instant case.

The request is ongoing. In the event that new materials responsive to this request are produced, discovered, or otherwise come into the possession of the prosecution or its agents,

said materials should be provided to the defendant without delay.

In the event that there is a charge for reproducing any of these materials please include an itemized list indicating the number of items (for

 example number of pages

of documents, number of photographs, X-ray films, number of CD-ROMs, etc.) and

the cost of copying per item.

1. Case file: Please provide a complete copy of the case file including all records

made by the laboratory in connection with this case.  If the file includes photographs, please include photographic quality copies. 

2. Laboratory Protocols:  Please provide a copy of all standard operating protocols

(SOPs) used in connection with the testing in this case.  To minimize any burden

of duplicating these items, we invite you to provide them in electronic form.

3. Chain of custody and current disposition of evidence: Please provide copies of all

records that document the treatment and handling of biological evidence in this

case, from the initial point of collection up to the current disposition.  This information should include documentation which indicates where and how the materials were stored (temperature and type of container), the amount of evidence

material which was consumed in testing, the amount of material which remains,

and where and how the remaining evidence is stored (temperature and type of


4. Software: Please provide a list of all commercial software programs used in the

DNA testing in this case, including name of software program, manufacturer

and version used in this case.

5. Macros:  If the results produced by the software are dependent on the instructions contained in macros, please provide copies of any macros used.  (For analyses performed with GeneScan and Genotyper, these macros are contained in

Genotyper output files in order to allow analysts to interpret the results.  Simply

providing a copy of the Genotyper output files in response to request 6 will satisfy this request as well).23

6. Data files: Please provide copies of all data files used and created in the course of

performing the testing and analyzing the data in this case. These files should include all data necessary to, (i) independently reanalyze the raw data and (ii) reconstruct the analysis performed in this case. For analyses performed with

GeneScan and Genotyper, these materials should include

(6.1) All collection files (such as injection lists and log files for an ABI 310


(6.2) All Genescan files, including sample files and project files.

(6.3) All Genotyper files, including templates/macros (see Request 5). 

7. STR frequency tables: Please provide copies of any allelic frequency tables relied upon in making statistical estimates in this case.  If the laboratory relied

upon published or publicly available data, this request can be satisfied by providing a specific reference to the source. 

8. Instances of Unintended DNA Transfer or Sample Contamination:  Please provide

copies of all records maintained by the laboratory that document instances of

unintended transfer of DNA or sample contamination, such as any instances of

negative controls that demonstrated the presence of DNA or the detection of

unexpected extra alleles in control or reference samples, and any corrective

measures taken.

9 Accreditation: Please provide copies of all licenses or other certificates of accreditation held by the DNA testing laboratory.

10. Laboratory personnel: Please provide background information about each person involved in conducting or reviewing the DNA testing performed in this case, including:

(10.1) Current resume

(10.2) Job description

(10.3) A summary of proficiency test results





Information compiled from the following sources:


DNA for the Defense Bar, National Institute of Justice

Evaluating forensic DNA evidence: Essential elements of a competent defense review
By William C. Thompson; Simon Ford; Travis Doom; Michael Raymer; Dan E. Krane

The Champion, April 2003


The Criminal Lawyer blog, Nathaniel Burney

DNA evidence:  good science, bad results

DNA for the Defense, 2010 presentation to WI SPD at fall conference, Keith Findley and Angela Williamson


This is the link to the .pdf file that explains the whole universe.