Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of biotechnology company Gilead Sciences (Nasdaq: GILD) plunged 15% Friday after it reported disappointing trial results for its experimental hepatitis C drug.

So what: Gilead said that most of the hepatitis C genotype 1 patients it treated with its GS-7977 drug relapsed after a course of treatment, triggering fresh concerns over the drug's efficacy in some patients. Naturally, the news bodes well for rival hep C treatment developers Vertex Pharmaceuticals (Nasdaq: VRTX) and Achillion Pharmaceuticals (Nasdaq: ACHN), whose shares are both up big today.

Now what: I'd look into this pullback as a possible buying opportunity. While Gilead was certainly dealt a setback, all companies in the hep C drug race will likely face the same challenge of treating the hepatitis C genotype 1 population with just one drug. Given the fact that it already has several drugs in its pipeline it can possibly combine with GS-7977, Gilead still seems like the smart bet to be the first to market.

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