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In many ways, Matt Fender, a 32-year-old resident of New York City, is the prototypical 23andMe customer: tech-savvy, educated, a bit of a worrier. But he wasn’t worried last December when he clicked a button to dump all the raw data from his 23andMe genetic test into a DNA search engine called Promethease, which sorts through data for gene variants that have received a mention in the medical literature.

Mr. Fender didn’t expect any revelations. He had already spent $5 on a Promethease report in 2016, which he’d found interesting but not life changing. But the company had recently emailed customers asking them to re-enter their data to be used for future research and quality control. In return, they were offered a free update.

Mr. Fender’s update included something new: the terms “PSEN1” and “pathogenic.”

Mr. Fender is a coder, not a geneticist, but he had spent enough time scrolling through his 23andMe results to know he had gotten some bad news.

The PSEN1 mutation is associated with an early-onset form of Alzheimer’s, and it is often described as “100...