DNA tests like 23andMe and AncestryDNA are becoming more and more popular. This raises privacy concerns — and not just for those taking the tests. Soon the concept of privacy, as we know it, will come to an end. Back in April, police arrested a man as the Golden State Killer, an infamous alleged serial killer who murdered 13 people in 1970s and 80s. After all this time, police found the suspect by uploading DNA from the crime scene to a genealogy website database and found a number of matches: his third and fourth cousins. At first, the police didn’t release a lot of these details. There was a lot of speculation about how they did find their man. But when I started calling up genealogists, they could guess right away. This is exactly how they have been finding biological parents of adoptees and sperm donor-conceived kids for years. Since then, the dam has broken. DNA is being used to solve cold cases and even active investigations. In the last 5 months alone, this technique has contributed to 15 arrests. You might wonder: were these alleged criminals unlucky? Did they just randomly happen to have family members who took a DNA test? The answer is maybe more disconcerting. A study found that most Americans of European descent can already be found through a relative’s DNA, just like the Golden State Killer. The relative can be as distant as a third cousin. Third cousins only share a small amount of DNA, but it’s enough to build family trees if you use data from the census, obituaries, and other public records. Investigators go through people in a family tree one-by-one until they get to their suspect. Police are doing this with a public genealogy database called GEDmatch, where anyone can upload results from 23andMe and AncestryDNA. Even if you never take a DNA test or commit a crime, this can affect you. Criminal cases are one thing. But here’s another scenario: A man donated sperm anonymously 30 years ago. If his second cousin once removed takes a DNA test, his children will be able to find him. Relatives you’ve never met can take DNA tests that affect you. And the actions you take can affect relatives that you don’t even know. If you think you have taken precautions to protect your privacy online, you could still find yourself out of luck. As DNA tests get more and more popular, databases like GEDmatch get bigger and bigger. Soon, it won’t be hard to imagine a world where anyone can be found for whatever reason through a relative’s DNA. The idea of your DNA being private will no longer exist.