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 biopharmaceutical giant Gilead Sciences (Nasdaq: GILD) rallied as much as 11% today following the release of new data for its hepatitis-C drug GS-7977 and after reporting fourth-quarter results.

So what: Gilead's gains today are all about the data released last night revolving around GS-7977. The study found that when the GS-7977 pill was combined with ribavirin, 100% of genotype 1 hep-C patients had no trace of the virus found after four weeks of treatment. Gilead also reported fourth-quarter results, which showed revenue grew by 10% to $2.2 billion, slightly beating consensus estimates of $2.18 billion, but earnings fell $0.08 shy of Wall Street's estimates ($0.97 vs. $1.05). Outside of the hep-C data, the highlight of Gilead's report was the 11% growth in its HIV drug, Atripla.

Now what: Today's data looks like bad news for much of the hep-C sector because, as of now, Gilead's drug looks far superior. That's the reason Idenix Pharmaceuticals (Nasdaq: IDIX) is dropping 15% today as it's still hoping to find a buyer of its experimental drug candidate IDX184. Then again, even with the hep-C market estimated at $5 billion, Gilead shares have tacked on more than 30% just since the start of the year, so what value may have been there is long gone. I'd consider taking another look at Gilead down the road, but only after a significant pullback.

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