Becker and Davey’s connection began 25 years ago, shortly after he was born. His mom agreed to donate his umbilical-cord blood, and for several years his cells lay frozen in a cord-blood bank in New York. Meanwhile, hundreds of miles away in Chicago, a 24-year-old Becker started getting fevers and night sweats and extreme fatigue—symptoms, it turned out, of non-Hodgkin’s lymphoma, a cancer that affects white blood cells. She went through multiple rounds of chemo. They all failed. “I was literally at the point where I was about to die,” she says. That’s when her doctor suggested a then-novel transplant involving cord blood from a stranger.

Read: When a DNA test reveals your daughter is not your biological child

Umbilical cords are rich in hematopoietic stem cells, which ultimately give rise to all different types of blood cells. To do the transplant, doctors would destroy Becker’s own cancerous cells before infusing her with hematopoietic stem cells from a healthy matched donor. Those cells would eventually divide to replace all the blood in her. It was a grueling procedure, and Becker did not fully recover for two years. But she has stayed healthy ever since, and she has always wondered over the years about the anonymous donor who saved her life. Her doctor told her the donor was untraceable. That’s how it’s supposed to work.

And when Becker finally spit into a tube for her AncestryDNA test, she was not trying to find her donor; she did not even know that was possible. She was just interested in her family history. But the test matched her to Patrick’s mom, Dania Davey, who immediately messaged Becker when she saw a new and very close match—so close as to be mother or daughter. Davey, who is adopted, had recently found her birth family through AncestryDNA, and this new match made no sense. She knew her birth mother. She certainly knew Becker was not her daughter. They both thought the test was wrong.

But Becker wondered about the transplant. She asked Davey whether she was from New York—because that was the only fact she knew about her cord-blood donor. Davey said yes. When Becker shared her hypothesis with her oncologist at Loyola Medicine, Patrick Stiff, he was initially skeptical. Transplants with hematopoietic stem cells—which can come from cord blood, as in Becker’s case, or from bone marrow—should only alter the blood cells in her body. The rest of her would still be her. Why would DNA in Becker’s saliva match her donor’s? (On the other hand, blood transfusions do not involve killing off the recipient’s blood cells and have only tiny, transient effects on the recipient’s DNA.)

Then, Stiff told me, he heard from yet another transplant patient, who spit into an AncestryDNA tube and got strange results. The DNA in saliva, it turns out, can come from cells in the cheek lining (which should have the recipient’s DNA) and from white blood cells (which should have the donor’s DNA) that guard against bacteria in the mouth. AncestryDNA and 23andMe actually advise customers who have had bone-marrow or cord-blood transplants against taking their tests, as the mix of genetic material can cause them to fail. Often enough, though, the tests return the genetic results of the donor. Many bone-marrow recipients have gotten their donors’ DNA results, and Stiff said he knew of at least one other unpublicized case of a cord-blood recipient who found their donor.