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 drug developer Exelixis (Nasdaq: EXEL) traded down slowly on Friday, punctuated by a brief mega-volume 10.4% collapse in the afternoon that lasted mere minutes.

So what: In the afternoon, a rumor of negative clinical trial data popped up. The quickly erased downward spike shows how valuable the cabozanitib prostate-cancer trials are to Exelixis' future prospects but also underscores how unsubstantiated the speculation really was.

Now what: That particular drug has run into issues before, taking the stock along for a slide. For example, Exelixis disclosed earlier this summer that some trial patients had died during treatment. But for a potential cancer-buster, that's kind of par for the course.

I hear echoes of Elan (NYSE: ELN) here, whose multiple sclerosis drug Tysabri also went through six kinds of hell before becoming the blockbuster that it is today. Our Rule Breakers team was confident that cabozanitib would make it to market last time I checked, and an unproven rumor of low tolerability shouldn't change that.

Interested in more info on Exelixis? Add it to your watchlist.

Fool contributor Anders Bylund holds no position in any of the companies discussed here, though he suffers from MS and thus follows Elan closely. The Motley Fool owns shares of Exelixis. Motley Fool newsletter services have recommended buying shares of Elan and Exelixis. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool is investors writing for investors.