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: Despite beating Wall Street's estimates for first-quarter results, shares of Elan (NYSE: ELN) plunged as much as 15% in early trading today. The stock has since recovered to be down about 4%.

So what: Elan earned $0.12 per diluted share on $313 million in Q1. Analysts had been calling for a $0.03 loss on $297.04 million, according to Yahoo! Finance data.

Now what: It didn't matter. Biogen Idec (Nasdaq: BIIB), an Elan partner in marketing the MS drug Tysabri, reported promising trial results for a competing MS treatment called BG-12. (Click here to read about Biogen's stock pop.) If approved, the new drug could cut into Elan's Tysabri sales because it's administered orally rather than intravenously. Tysabri patients are also regularly screened for a rare brain disease called PML, which can manifest as a side effect.

Investors' fears of Tysabri, which accounted for 78% of Elan's first-quarter revenue, being replaced may be justified someday. For now, it's important to remember that Elan easily beat estimates while Biogen's BG-12 remains in a phase 3 trial. Mr. Market is likely overreacting. 

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