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What: Shares of Idenix (Nasdaq: IDIX) are plunging today, down by 16% at the low, after competitor Pharmasset (Nasdaq: VRUS) discontinued use of one of its experimental drugs due to safety concerns.

So what: Pharmasset's experimental hepatitis C drug PSI-938 reported complications with liver function, although the developments aren't expected to interfere with Gilead Sciences' (Nasdaq: GILD) planned acquisition of the company. Idenix also has a hepatitis C drug in its pipeline, which is the company's current focus.

Now what: The news is causing concern among some hepatitis C drugmakers, including Inhibitex (Nasdaq: INHX), which is also seeing downside today. Interestingly, rival Vertex Pharmaceuticals (Nasdaq: VRTX) is seeing healthy gains, with setbacks for PSI-938 considered positive for Vertex. Idenix's and Inhibitex's hepatitis C drugs are more similar to Pharmassets, which is causing investors to flee for cover.

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Fool contributor Evan Niu holds no position in any company mentioned. Click here to see his holdings and a short bio. Motley Fool newsletter services have recommended buying shares of Gilead Sciences and Vertex Pharmaceuticals. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.