This week, we filed an amicus brief in a South Dakota case arguing that the Fourth Amendment prohibits the police from surreptitiously collecting our DNA without a warrant. This case is one of the first to challenge the collection of DNA from a free person after results of a genetic genealogy database search linked her to a crime.

In the case, prosecutors have charged Teresa Bentaas, a lifelong Sioux Falls resident, with first degree murder for abandoning a newborn 39 years ago. To link Ms. Bentaas to the baby, police secretly collected her DNA from items they found in her trash, extracted and sequenced it, and then compared it to the baby’s. Ms. Bentaas was not under arrest or in police custody at the time they collected and searched her DNA, so, unlike an arrestee, there can be no argument her Fourth Amendment rights were in any way “diminished.”

Ms. Bentaas’s attorneys have filedmotion to suppress the DNA evidence and all evidence collected by the police after that, arguing this secret and warrantless rummaging and data collection violates the Fourth Amendment and South Dakota’s state constitutional equivalent. EFF, joined by the ACLU and ACLU of South Dakota, filed a brief in support.

Prosecutors claim that the Fourth Amendment doesn’t apply in this context because Ms. Bentaas abandoned her privacy interest in her DNA when she left it behind on the items she threw out in her trash. However, we argue the Fourth Amendment creates a high bar against collecting DNA from free people, even if it’s found on items the person has voluntarily discarded. In 1978, the Supreme Court ruled that the Fourth Amendment does not protect the contents of people’s trash left for pickup because they have “abandoned” an expectation of privacy in the trash. But unlike a gum wrapper or a cigarette butt, our DNA contains so much private information that the data contained in a DNA sample can never be “abandoned.” Even if police don’t need a warrant to rummage through your trash (and many states disagree on this point), Police should need a warrant to rummage through your DNA.

A DNA sample—whether taken directly from a person or extracted from items that person leaves behind—contains a person’s entire genetic makeup. It can reveal intensely sensitive information about us, including our propensities for certain medical conditions, our ancestry, and our biological familial relationships. Some researchers have also claimed that human behaviors such as aggression and addiction can be explained, at least in part, by genetics. And private companies have claimed they can use our DNA for everything from identifying our eye, hair, and skin colors and the shapes of our faces; to determining whether we are lactose intolerant, prefer sweet or salty foods, and can sleep deeply; to discovering the likely migration patterns of our ancestors and the identities of family members we never even knew we had.

Despite the uniquely revealing nature of DNA, we cannot avoid leaving behind the whole of our genetic code wherever we go. Humans are constantly shedding genetic material; In less time than it takes to order a coffee, most humans lose nearly enough skin cells to cover an entire football field. The only way to avoid depositing our DNA on nearly every item we touch out in the world would be to never leave one’s home. For these reasons, as we argue in our brief, we can never abandon a privacy interest in our DNA.

The Bentaas case also raises thorny Fourth Amendment issues related to law enforcement use of genetic genealogy databases, which South Dakota police used earlier in their investigation to try to find a genetic connection to the deceased baby. We’ve written about these issues before. In the Bentaas case, the police exhumed the body of the infant, extracted a DNA sample from the remains, and then worked with a private company called Parabon Nanolabs to search through the consumer genetic genealogy database GEDmatch, to try to find a connection between the infant’s DNA and GEDmatch users. Parabon wasn’t able to find a close relative but did identify two individuals who could have been between sixth to eighth degree relations. A police officer then did his own research on public data websites to try to find a potential suspect. He settled on Ms. Bentaas and her husband, both potential biological matches, and then surreptitiously collected DNA samples from items he found in their trash.

This process of searching genetic genealogy databases in criminal investigations has become quite common. More than 26 million people have used genetic genealogy databases like GEDmatch to identify biological relatives and build a family tree, and law enforcement officers have been capitalizing on all that freely available data in criminal investigations across the country. Estimates are that genetic genealogy sites were used in around 200 cases just last year. For many of those cases, like this one, officers never sought a warrant or any legal process at all before searching that private database.

Police access to this data creates immeasurable threats to our privacy. It also puts us at much greater risk of being accused of crimes we didn’t commit. For example, in 2015, a similar forensic genetic genealogy search led police to suspect an innocent man. Even without genetic genealogy searches, DNA matches may lead officers to suspect—and jail—the wrong person, as happened in a California case in 2012. That can happen because our DNA may be transferred from one location to another, possibly ending up at the scene of a crime, even if we were never there.

Even if you yourself never upload your genetic data to a genetic genealogy website, your privacy could be impacted by a distant family member’s choice to do so. Although GEDmatch’s 1.3 million users only encompass about 0.5% of the U.S. adult population, research shows that their data alone could be used to identify 60% of white Americans. And once GEDmatch’s users encompass just 2% of the U.S. population, 90% of white Americans will be identifiable. Other research has shown that adversaries may be able to compromise these databases to put many users at risk of having their genotypes revealed, either at key positions or at many sites genome-wide.

This is why this case is so important—and why we need strong rules against police access to genetic genealogy databases. Our DNA can reveal so much about us that our genetic privacy must be protected at all costs. We hope the South Dakota court and other courts addressing this issue will recognize that the Fourth Amendment protects us from surreptitious collection and searches of our DNA.