>>Moreover most of the key top 5 institutional owners of ALXN per 2Q '15 did sell a large % of their ALXN holdings even BEFORE 7/23/15 OMER's FT on OMS721
Nothing to do with OMER I believe. If these guys were selling because they were scared of OMER's project, they would have stepped up and bought OMER at some point during the recent weakness.
There are a lot of players that tend to short biotech stocks where they believe a recently approved product will show an anemic growth pattern for the first several quarters. Likely most of those guys have never even heard of aHUS and got blindsided here.