Of all the cancers in the world, lung cancer is the deadliest. This year, the National Cancer Institute estimates it will kill 150,000 people, despite a growing number of more targeted therapies. Part of the problem is there are a lot of different mutations that can cause the most common form of it: non-small cell lung cancer. And while the genetic profile of a tumor can tell doctors which treatment to prescribe, finding the mutation is an exercise in trial and error. Screen for the most common mutation, then try that drug. If it doesn’t work, screen again and try something else. It can take weeks or even months to find a treatment that clicks, taking time patients don’t have.

But that could all soon change. On Thursday, the Food and Drug Administration approved the first next-generation-sequencing-based test, from Thermo Fisher Scientific, that can tell you how different drugs will work for you, based on the genetic makeup of your tumor. And it only takes four days to get back results. In many ways, it represents the leading edge of precision medicine’s maturation from a buzzword in grant applications and investor pitch decks to a real, workable product that can actually improve patient outcomes.

Getting the FDA’s approval took nearly two years and 220,000 pages of data. (That’s like reading Karl Ove Knausgaard’s 6-book autobiographical memoir front to back 61 times in a row. Talk about My Struggle.) But the process has helped clarify the agency’s thinking about how to regulate personalized treatments going forward, opening up doors for tech that's still in the pipeline.

Oncomine Dx Target Test and the PGM Dx System it runs on. The technology can tie multiple genetic markers to approved cancer drugs using just 10 nanograms of DNA. Thermo Fisher Scientific

The panel, called Oncomine Dx Target Test, takes a tiny amount of tumor tissue and reports on alterations to 23 different genes. All that information is useful for physicians, but three in particular—ROS1, EGFR, and BRAF—are the the most crucial. That’s because those mutations have drugs to match: Precision medicine chemotherapies from Pfizer, Novartis, and AstraZeneca. The test can be performed at any CLIA-certified lab, and it’s already being offered by two of the largest oncology-focused ones.

Getting the FDA to approve that amalgam of tests wasn’t easy. “Putting multiple genes and multiple drugs on the same test; all of these are firsts,” says Joydeep Goswami, Thermo Fisher’s president of clinical next generation sequencing. “That put the technology under extraordinary scrutiny.” The FDA usually approves one diagnostic for one product or drug—that’s it. But the whole point of precision medicine is to tailor treatments for patients based on their genes, and a bunch of one-off genetic tests aren’t going to deliver on that promise. So a multi-gene, multi-drug panel is kind of a big deal.