There will be the inevitable margin calls and knee-jerk responses to those who saw it down yesterday so much and some will sell without ever even knowing what drove it down.
Then, once things calm down, the bottom feeders will come in and see the tremendous value that is still represented and it will claw back much of what it lost.
I agree with you that it will dip into the $19s briefly and that should be about it. Those that buy at the 19-20 levels will get bragging rights for years when it bounces back.
Also agree with the market cap of $2.4B is almost comical, but again, once things calm down and the cooler heads look at the value of the company versus the stock price/market cap - the big money will move in strongly.
Don't let the clowns fool you by pretending it is going down far more than it is. The value of this company far exceeds the one drug that didn't proceed. The cash flow is excellent for a biotech, the cash position is strong, and while the GSK is a setback to be sure, in time it will be revealed to be a minor setback.