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 Chinese advertising company China MediaExpress (Nasdaq: CCME) got crushed today, falling as much as 34% in intraday trading on huge volume.

So what: And here we have yet another short-seller target in the battlefield that is Chinese small caps. China MediaExpress' shares started to sag after a pair of short-seller reports from Citron Research and Bronte Capital -- between Jan. 28 and Feb. 2 the stock lost 20% of its value. Some long investors have been pushing back, an effort that included a rebuttal posted yesterday on Forbes' website. But shares simply buckled today after short-seller Muddy Waters piled into the fray. Chinese small-cap investors will be all too familiar with Muddy Waters as the outfit that skewered Rino International, a stock that's now trading in pink sheet ignominy.

Now what: As with all of the under-fire Chinese small caps, the trick with China MediaExpress is figuring out who has the facts. At this point, the short-sellers seem to have the upper hand in the market, and even the suggestion that there's impropriety at one of these companies is enough to send shares tumbling. Shares of China MediaExpress are now trading at less than four times what the company is expected to report in 2011 earnings per share. Of course that doesn't mean much if it turns out that it's been fudging the numbers.

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Fool contributor Matt Koppenheffer does not own shares of any of the companies mentioned. You can check out what Matt is keeping an eye on by visiting his CAPS portfolio, or you can follow Matt on Twitter @KoppTheFool or on his RSS feed. The Fool's disclosure policy prefers dividends over a sharp stick in the eye.