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 hepatitis C drugmaker Achillion Pharmaceuticals (NASDAQ:ACHN) sank by more than 20% at one point today. Although there was no specific catalyst driving the stock's downfall, it appears to be cratering as a result of the bevy of disappointing news emanating from fellow hep C drugmakers AbbVie (NYSE:ABBV), Gilead Sciences (NASDAQ:GILD), and Merck (NYSE:MRK)

AbbVie kicked off the streak of hep C drugmaker disappointments with its earnings report earlier this week. In the report, management stated that the commercial uptake of its drug, Viekira Pak, will take longer than expected, and sales probably won't reach the Street's lofty estimates of nearly $3 billion in 2015.

Gilead added to the misery yesterday on its earnings call, giving annual sales estimates for its hep C franchise of Harvoni and Sovaldi that fell short of some estimates by a staggering $1.5 billion. 

And in a rare move, the Food and Drug Administration rescinded its Breakthrough Therapy designation for Merck's hep C combo treatment consisting of grazoprevir/elbasvir today. While the FDA didn't provide any details on the decision, it appears that the agency no longer views additional hep C drugs as a major priority, hurting Achillion's chances of garnering an accelerated review for its offerings down the road. 

So what: Achillion is a one trick pony in many ways, and this avalanche of bad news in the hep C market would appear to limit the stock's upside potential. Put simply, this clinical-stage biotech appears to be getting into the game too late, and additional drugs won't receive any favors from the FDA moving forward.

Now what: Achillion's shareholders were hoping for a buyout this year, but that no longer appears to be a likely scenario. AbbVie, Gilead, and Merck are all working on new hep C combos of their own that would shorten treatment times without the need for interferon, meaning that Achillion's pipeline probably offers little value for these market leaders at this point.