Relax, the news is 20 minutes old. Once the Wall Street experts digest it, up their ratings and then realize Tysabri will get a bit more than 15% of the MS market (like 85% more) you might see some upward revisions.
Watch the bid vs. ask volumes, people are putting out offers to buy 10k+ shares at a time, and selling in the hundreds. The sellers are likely selling into the jump without realizing why it's moving.
Who knows, maybe the really smart money knows it will go back down before making a run up, but I'm way to small an investor to take that chance. I don't see this sitting below $20 for long, remember before the problems with Tysabri it was a $30 stock.