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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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