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: InterMune (Nasdaq: ITMN) dropped 15% in intraday trading today after a major shareholder indicated it might sell its stock in the company.

So what: Warburg Pincus, which owns 16% of InterMune and has representation on InterMune's board of directors, indicated in an SEC filing that it is considering selling some or all of its position. According the filing, the sale could take place as early as the second quarter. Having an employee on InterMune's board could give Warburg Pincus an unusually strong understanding of the biotech company's outlook.

Now what: InterMune's stock soared from less than $15 in December to more than $50 in April before closing yesterday at $43.24. The increase has been driven by news surrounding Esbriet, its experimental treatment for a terminal lung disease. Don't let the reasonable 18 times P/E ratio fool you though; the company's only profitable quarter was the fourth quarter of 2010, when it benefitted from a rights sale to Roche and terminating an agreement with Hoffmann-La Roche.

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Fool contributor Cindy Johnson does not own shares of any company named above. 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.