Biotech investors are never safe from the quiet robber of wealth -- dilution. Although too many managers in the sector see raising cash at the expense of shareholders as relatively victim-less, oftentimes debt markets just aren't an avenue available to these companies. But even investors who do their homework and feel good about a company's financial position can be caught off guard.

In this video, health-care analyst David Williamson takes a closer look at one such case, Isis Pharmaceuticals. Shares plunged 10% after a surprise announcement to raise $170 million. Watch and find out the reasoning behind Isis' surprise announcement  and what investors should make of it.