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.

David Williamson has no position in any stocks mentioned. Follow David on Twitter: @MotleyDavid.

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