Apellis Pharmaceuticals shares rose 136% to $40.30 early Tuesday, their biggest ever one-day gain. The surge followed Biogen’s $5.6 billion acquisition announcement, valuing Apellis at $41 per share—140% above Monday’s close of $17.09. This reversed a year in which the stock fell nearly a third, including a 25% drop in the past 12 months.

Apellis shareholders will receive $41 per share in cash, plus a right to two extra payments of $2 per share each if Syfovre, the company’s treatment for geographic atrophy, achieves certain yearly sales targets. The deal structure underscores two main risks: First, there is significant uncertainty about Syfovre’s ability to meet these sales targets due to recent declines in sales and rising competition. Second, if these targets are not met, Apellis shareholders will not receive the additional payments. Biogen recognizes these challenges and has structured the deal so that current Apellis holders benefit only if Syfovre’s sales improve beyond current levels.

The market responded less favorably to Biogen. Its shares fell 4.9% as the S&P 500 rose 1.3%. Investors questioned Biogen’s valuation, given Apellis’s recent results: total revenue dropped 6% last quarter, and Syfovre sales fell 8% year over year amid new competition. Analysts also cite slow growth and competition for Syfovre and Empaveli, Apellis’s main products, which generated $689 million in revenue last year. Biogen expects immediate revenue contributions and projects mid-to-high-teens growth through 2028. The buyout also accelerates Biogen’s nephrology expansion and reinforces its strategic focus on immunology and rare disease, a segment where Q4 sales fell 16%.

Biogen is making this deal from a strong position. Its shares are up 6.6% this year and about 42% over the past twelve months. Biogen beat the S&P 500 both times. This growth is thanks to strong fourth-quarter earnings and rising sales of Leqembi, its Alzheimer’s drug made with Eisai. The main question for traders now is whether Biogen can stop Syfovre from losing ground to competitors under its new ownership. Another key question is whether the high price paid will be worth it based on the revenue growth Biogen expects through the end of the decade.