Christmas came early for Merck this year. Its immuno-oncology drug, Keytruda, received FDA approval in first-line non small cell lung cancer late Monday, two months before its scheduled decision date of December 24.

The approval allows new patients to be treated with Keytruda if their tumors express high levels — greater than or equal to 50% — of PD-L1, a biomarker and important bellwether for how the drug will perform.

The FDA also broadened Keytruda’s label, approving the drug for use in patients whose tumors express any level of PD-L1 in the second-line setting.

The new approvals put Keytruda in a market-leading position compared to Bristol-Myers Squibb’s Opdivo. Until now, a significant differentiator between the therapies was the requirement that patients taking Keytruda first undergo PD-L1 testing.

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Opdivo’s second-line dominance is now under siege, as PD-L1 testing will become more common in new patients, making it more likely that physicians will prescribe Keytruda in that setting, Credit Suisse analyst Vamil Divan wrote in an investor note on Monday. “We believe the increasing familiarity physicians (and their staff) will have with using Keytruda in first-line non small cell lung cancer will help them also dominate the second-line market in patients who have tumors with PD-L1 expression levels of 1% to 50%,” he wrote.

Merck said Tuesday that sales of Keytruda jumped 124% to $356 million in the third quarter of 2016, compared to the same quarter a year ago. The drugmaker has said it currently has 10% market share of second-line lung cancer in the U.S.

The market value of treating first-line lung cancer is estimated to be worth $12 billion, according to Evercore ISI analyst Mark Schoenebaum, who wrote in a research note that he believes Keytruda’s label allows it access to up to 30% of that market. Merck has estimated that between 25% to 30% of patients with non-small cell lung cancer have levels of PD-L1 in their tumors that would make them eligible for treatment — and that approximately 60% of U.S. physicians who treat the disease are already currently testing for PD-L1. The company is expected to dominate the first-line setting until mid-2017, when combination therapies could threaten its market position, according to John Scotti, another Evercore ISI analyst.

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The approval is a huge boon to Merck, which now markets the only immuno-oncology drug approved by the FDA to improve survival in first-line lung cancer patients. After the company released positive data at the European Society for Medical Oncology meeting earlier this month, analysts considered the drug a shoo-in for this approval but the decision was not expected until December.

The immuno-oncology market has been upended in recent months. In early August, Bristol-Myers Squibb’s Opdivo was in pole position — leading treatment among PD-1 inhibitors in sales for second-line lung cancer, due in part to Keytruda’s diagnostic testing requirement for PD-L1 levels. But BMS suffered a setback in clinical development; Opdivo missed the primary endpoint of its monotherapy trial for first-line lung cancer. That misfire was largely attributed to a risky bet in the trial: including a wide range of PD-L1 expressing patients in an attempt to receive the broadest possible label.

Opdivo has been the market leader, based on sales. Opdivo brought in $942 million in revenue in 2015, compared to Keytruda’s $566 million.