This approval’s importance is hard to overstate. For one, lung cancer is the second most commonly diagnosed cancer in America (behind breast cancer) and by far the deadliest one, killing nearly 156,000 U.S. residents every year. The specific type of lung cancer that Keytruda plus chemo is now approved to treat—non-squamous non small cell lung cancer (NSCLC)—makes up about three in four of the more than 222,000 new annual lung cancer cases in America.

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That adds up to a truly gigantic market opportunity and potential billions upon billions in sales (oh, and also years added to patients’ lives). “This is the key game changer for Keytruda,” as Leerink Partners analyst Seamus Fernandez put it, according to Reuters.

The treatment was actually already approved as a “first-in-line” therapy (i.e., it could be the first drug a doctor prescribed to a patient) for advanced lung cancer—but with one big caveat: patients had to have high levels of a certain protein targeted by Keytruda called PD-L1. This latest FDA green light nixes that constraining requirement.

Keytruda’s newest approval also sets it apart from rivals like Bristol-Myers Squibb’s Opdivo, another so-called “checkpoint inhibitor” immunotherapy drug. That treatment failed to meet its goals in recent lung cancer trials, paving the way for Keytruda to potentially dominate the field and sending Bristol-Myers shares tumbling.

It’s a marked reversal of fortune. Opdivo once seemed like it was fated to become the next-gen cancer drug king, easily lapping Keytruda in sales. And, to be fair, it could still fare very well: both Keytruda and Opdivo are being tested in dozens of cancer types and have racked up approvals in several of them. (Pfizer, AstraZeneca, and Roche also have their own checkpoint inhibitor drugs.) But the lung cancer market is key—and there, at least, Keytruda has thrown down the gauntlet.