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What's happening?
Shares of Exelixis (NASDAQ:EXEL) have leapt 14% higher on Wednesday, with no apparent news-based driver behind the surge.

Why it's happening
Exelixis' most recent news came three weeks ago, when the small biotech company extended a European distribution agreement for cancer drug Cometriq. The agreement, which covers Cometriq's distribution in the European Union, Switzerland, Norway, Turkey, and Russia, was originally set to expire at the end of this year. It has now been extended to the end of 2019, and will now change from a fixed-fee structure to a sales-based margin setup, which could wind up bringing in more reliable revenue to Exelixis. This news drove a surge in Exelixis' stock in early January, and it appears likely that today's move could be some short-covering movement in the absence of negative price pressures, as a quarter of Exelixis' shares were sold short as of this writing. Exelixis could have also gotten a delayed boost from coverage of Array Biopharma's deal for the rights to another cancer treatment drug, ecorafenib. The relation here is that Exelixis' cobimetinib is in trials as a combination with Roche's Zelboraf to treat melanoma, which is similar to the drugs that Array now controls following its rights reacquisition.

Investors won't get much news on cobimetinib this year, as its Roche combo is slated for approval later this year, but they could see the results of the reworked Cometriq agreement once first-quarter results come in. There's still time for ample volatility between now and then, as shown by today's unexpected (and thus far unexplained) move.