DNA testing has done more to expose wrongful convictions than any other single development in the history of American criminal justice. Since the Innocence Project began using DNA to challenge convictions in the early 1990s, more than 375 people have been exonerated through DNA evidence - people who had collectively spent over 5,000 years in prison for crimes they didn't commit.
But DNA is not a magic wand. It's not available in most cases, it's not automatically granted when requested, and even when testing produces favorable results it doesn't automatically produce freedom. Understanding what DNA testing actually involves - and its limitations - is essential for families trying to figure out whether it's an option.
When DNA Testing Is Possible
DNA testing is only relevant when biological evidence was collected and preserved. This means the crime involved some form of biological material - blood, semen, hair with root attached, saliva, skin cells - that was collected during the investigation and can still be located.
Not every crime scene yields biological evidence. Many serious crimes - some murders, most robberies, many assault cases - have no biological evidence at all. In those cases, DNA testing is simply not a tool that's available.
Even when biological evidence was collected, it may not have been preserved. Evidence can degrade, be destroyed, or be lost over years and decades. Before pursuing a DNA testing motion, families need to determine whether the evidence still exists and is in a condition that can produce results.
What DNA Testing Can Show
DNA testing compares a biological sample from the crime scene to a known DNA profile. If the crime scene sample contains DNA, that DNA can be profiled and compared to the convicted person's DNA profile. If the profiles don't match - if the DNA from the crime scene is from someone other than the convicted person - that's powerful evidence of innocence.
It's important to understand what this means legally. DNA evidence that excludes the convicted person doesn't automatically vacate the conviction. It creates grounds for a post-conviction petition claiming newly discovered evidence of innocence. A court then has to evaluate that evidence in the context of the entire case and determine whether it would have produced a different verdict.
In most cases, favorable DNA results do lead to exoneration - particularly when combined with identification of the actual perpetrator through a CODIS database match. But the legal process still has to play out.
How to Request Post-Conviction DNA Testing
Almost every state now has a statute allowing convicted persons to petition for post-conviction DNA testing. The specific requirements vary, but generally the petitioner must show that the evidence exists and is suitable for testing, that DNA testing wasn't available at trial or wasn't performed, that the results would be material to the conviction, and that the testing is being requested for legitimate purposes and not merely to delay proceedings.
Some states require that the person maintain their claim of innocence. Others allow DNA testing requests even from people who pleaded guilty, recognizing that guilty pleas sometimes occur for reasons unrelated to actual guilt.
These motions are typically filed by attorneys. Innocence organizations with legal staff can handle the petition process. Going through the process without legal representation is possible but significantly harder.
When DNA Evidence Already Exists but Wasn't Used at Trial
In some cases, biological evidence was tested before or during trial - but the results were inconclusive, or the evidence was tested using older technology that couldn't produce a definitive profile. Advances in DNA technology, including touch DNA analysis and probabilistic genotyping, have made it possible to generate profiles from samples that previously couldn't be analyzed.
If biological evidence was tested at trial but the technology has improved, a motion for retesting can argue that current methods would produce results unavailable to the original jury.
Cases Where DNA Won't Help
For cases without biological evidence, DNA is not the answer. The wrongful conviction cases that don't involve DNA - which is the majority - have to be fought through other avenues: challenging eyewitness identification, uncovering Brady violations, finding witnesses who didn't testify, or challenging forensic evidence in other categories.
The absence of DNA evidence doesn't make a case hopeless. It just means the strategy has to be different. Many of the most prominent exonerations in recent years have come in non-DNA cases - through the work of organizations like the Michigan Innocence Clinic, which focuses specifically on cases without biological evidence.
Use our state directory to find innocence organizations in your state, including those that work on both DNA and non-DNA cases.