On Monday, SAC disclosed they own a 5.1% "passive" stake (4,882,700 shares) in the company. This is up from the 1,412,000 share stake the firm disclosed at the quarter ended March 31, 2007. In late April, Citadel LP disclosed a 5.52% "passive" stake (5.575 million shares) in the company.
We don't know why they are buying up the stock. But does it really matter? Apparently it sees something, and it would be safe to assume the stock has a good chance of outperforming.
BMRN 1-yr chart