The Limitations of DNA Evidence in Florida Criminal Cases

September 5, 2025 Don Pumphrey, Jr. Criminal Defense Social Share

In Florida, DNA evidence is often used as proof that someone is guilty of a criminal offense. Many may be inclined to believe that if a DNA “match” to a defendant occurs, this equates to someone having committed the charged crime. But often, this is not true.

In many Florida criminal cases where DNA evidence is used to demonstrate someone’s guilt, there are serious limitations to what this actually tells us. This is because there are many potential issues with the use of DNA evidence – including its transferability, mishandling or contamination of samples by law enforcement, and more. 

This is not to say that DNA is never reliable in a criminal case. But a frequently asked question is – what are the limitations of DNA evidence in Florida criminal cases? Moreover, can any of these limitations be used to exclude the DNA evidence from being introduced in court? This blog will explore this key topic.

In evaluating the probative (evidentiary) value of a DNA sample in a criminal case, there are various factors that are crucial to consider. These include:

• Type of DNA discovered: If A and B are coworkers, finding some skin cells of A on B would not necessarily indicate A’s guilt if accused of sexual battery of B. However, if A claims he has never had sex with B and sperm cells are found during a rape kit, this is more indicative of B’s guilt.
• Transferability: DNA is extremely transferable. Someone may shed DNA (e.g. skin cells) simply by walking around a location or touching a common surface (e.g. a doorknob). If people who occupied the same space have each other’s DNA on or near them, this is often not a “smoking gun” due to transferability.
• Chain of custody: From the time DNA is initially collected to the time it is used in a criminal proceeding, law enforcement must properly document who had custody of the DNA sample and how it was used/stored. Lapses in the chain of custody (e.g. missing entries) may lead to a tampering or contamination challenge, which may result in its exclusion from court.
• Method or place of collection: DNA must be collected using sterile tools, gloves, masks, and sealed evidence containers to avoid the sample’s contamination. Moreover, samples collected in certain areas (e.g. a crowded bar) are less likely to be reliable, especially if they contain multiple samples due to shedding.
• Contextual relevance: If someone’s DNA is found in an expected place (e.g. in a friend’s home or in their car), this likely has little to no evidentiary value. But in some cases (e.g. if DNA is found at a crime scene in a city the person claims they have never visited), this is more indicative of guilt.
• Timing of DNA deposit: Just because DNA is in a location does not mean it got there recently. For example, in State v. Sephes, 262 So.3d 811 (Fla. 4th DCA 2019), the court noted that DNA on a gun alone could not be used to prove Sephes illegally possessed it after he committed a felony. The court reasoned that Sephes’s DNA could have remained on the gun since before he was prohibited from possessing it.
• Random match probability: DNA matches are often accompanied by a probability statement (e.g. “a 1 in 1 million chance of a random match”). 

The last of these, random match probability, is one of the most frequently misunderstood aspects of DNA’s use in criminal cases. If someone hears that there is a “1 in 100 chance” of a random DNA match, the immediate assumption is that there is only a 1 in 100 chance that the defendant did not commit the crime.

However, what 1 in 100 really means is that in a city of 1 million people, 10,000 would also come back a positive match for the DNA (1 in 100). Unless there is additional corroboration of the defendant’s guilt, this actually means the chances of guilt – not innocence – are quite low. It is essentially the equivalent of saying “the perpetrator had red hair, and the defendant has red hair, so the defendant is guilty.”

It is true that the lower the odds of a random match (e.g. odds are 1 in 5 billion), the more likely a defendant was at a particular scene. But this is not inherently criminal, especially if DNA is discovered in a location that is unsurprising given transferability or a defendant’s daily habits (e.g. someone’s skin cells in an alley near their office building). 

Thus, it is important to understand that for DNA to be truly probative (strongly indicative of the defendant’s guilt), there must be DNA of a type not expected to be found (e.g. sperm cells on a woman the defendant claims they have never met), as well as low odds of a random match (“1 in a billion” instead of 1 in 100, etc.).

Examples of when DNA is highly probative (indicative of guilt) may include:

• C claims never to have met D, who lives in another city, but his DNA is found on her in a battery case with a 1 in 10 billion chance of a random match
• F claims never to have met G, but his sperm cells are found inside G during a rape kit with a 1 in 500 billion chance of a random match
• H claims to have never owned a gun nor been to Florida. However, his DNA is found on a gun abandoned at a crime scene in Tampa, with a 1 in 5 billion chance of a random match

However, examples of when DNA are not indicative of guilt (even though its presence may seem to be strong evidence at first) include:

• G legally sells T a firearm. A shooting occurs involving the firearm. Police swab the gun and find DNA that matches G’s. Police suspect G and arrest him.
• L’s DNA is found on a kitchen knife he uses to cook before selling it. The same knife is then used to murder someone. L is charged, as the presence of touch DNA is considered evidence of his guilt.
• Passengers frequently ride in M’s work car, leaving behind their DNA. Later, someone commits a robbery and steals M’s work car to do it. Police find the abandoned vehicle, swab it, and discover M’s DNA on the steering wheel and door handles. Investigators consider this as evidence of M’s involvement, and he is arrested.

Because DNA is not a magic wand that can be simply waved to obtain a conviction, there are many grounds upon which a Florida criminal defense attorney may move to exclude DNA evidence. Understanding the limitations of DNA evidence, from how it is collected and stored to its transferability, strengthens the grounds for a motion to exclude.

Grounds for a motion to exclude DNA evidence from being used against a defendant in Florida criminal court may include:

• Daubert challenges: Under Fla. Stat. 90.702, the methods of collection, storage and testing of DNA (as well as expert testimony relating to it) must be sufficiently reliable. If the methods are unreliable (e.g. untested) or the “expert testimony” is too speculative, it may be excluded. 
• “DNA alone”: DNA can only serve as the basis for a conviction if every “reasonable hypothesis of innocence” is excluded by its presence. If the only evidence against a defendant is the presence of DNA on an item (no witnesses, no surveillance footage, etc.), a motion to exclude or even to dismiss may be appropriate. State v. Sephes, 262 So.3d 811 (Fla. 4th DCA 2019)
• 90.403 challenges: Even relevant evidence in Florida is to be excluded if its probative (evidentiary) value is substantially outweighed by the danger of unfair prejudice, confusion, misleading the jury, or needlessly cumulative presentation of evidence (Fla. Stat. 90.403). This may be relevant in cases where DNA’s presence is unsurprising (e.g. in the defendant’s home) but the State still seeks to use it as evidence of guilt.
• Chain of custody challenges: A flawed chain of custody will not necessarily result in DNA’s suppression. However, if the defense alleges probable tampering/contamination based on specific facts, the State must refute this. Otherwise, the DNA is excluded. Murray v. State, 838 So.2d 1073 (Fla. 2002)
• Fruit of the poisonous tree: If DNA evidence is obtained without a warrant and is the product of force, coercion or threats (or the contents of the warrant were false or overly broad), DNA evidence may be excluded, as it was unconstitutionally obtained (“fruit of the poisonous tree,” per Wong Sun v. United States, 371 U.S. 471 (1963)). 

In sum, there are various limitations to DNA evidence in establishing guilt in a Florida criminal case. These include transferability, mixed or contaminated samples, misunderstandings of what “random matches” truly mean, and more.

Though DNA can sometimes be highly probative of guilt, it may not be. Depending on the facts of a particular case, a Florida criminal defense attorney can move to exclude DNA evidence on many grounds from being used against a defendant – or even move to dismiss the case if no other evidence indicates guilt.

If someone is arrested and formally charged in Florida with an offense and is concerned about DNA evidence, it is critical to find experienced and trusted legal representation as soon as possible. This decision could make the difference in whether or not someone faces a lengthy prison term and hefty fines.

Criminal Defense Attorney in Tallahassee, FL

Don Pumphrey, Jr. is a Former Prosecutor, Former State Police Officer, Lifetime Member of the Florida Association of Criminal Defense Lawyers; for over 25 years as a private defense attorney who is Trusted, Experienced, Aggressive in Criminal Defense as a Trial Attorney, Criminal Lawyer, Criminal Defense Lawyer for the accused in Florida State Courts located in Tallahassee, Florida but handling cases throughout the State of Florida.

Don Pumphrey, Jr. and the Tallahassee criminal defense lawyers at Pumphrey Law have decades of experience fighting drug charges on behalf of clients and winning. Call Pumphrey Law now at (850) 681-7777 to learn more about what we can do for you. Our lawyers will be happy to provide you with a free consultation.

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