Shares of Nightstar Therapeutics PLC rocketed 67% to pace all premarket gainers in very active trade, after the gene therapy company agreed to be acquired by Biogen Inc. /zigman2/quotes/201531540/composite BIIB -0.32% in a cash deal valued at about $877 million. Trading volume in Nightstar spiked to over 570,000 shares, compared with the full-day average of about 71,300 shares. Under terms of the deal, U.K.-based Nightstar shareholders will receive $25.50 for each Nightstar share they own, which represents a 68% premium to Friday's closing price of $15.16. The deal is expected to become effective by mid-year 2019. "This transaction accelerates treatment to patients through Nightstar's key retinal gene therapy programs that modify or halt progression of blindness," said Nightstar Chief Executive David Fellows. Nightstar shares have climbed 15.7% over the past 12 months, while the iShares Nasdaq Biotechnology ETF /zigman2/quotes/206189322/composite IBB +0.51% has gained 5.0% and the S&P 500 /zigman2/quotes/210599714/realtime SPX -1.12% has advanced 4.2%.