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 China MediaExpress (Nasdaq: CCME) plunged today on high volume, after Citron Research opined the stock was "too good to be true."

So what: Citron asserted that China Media's growth and profits -- on a similar order as Focus Media's (Nasdaq: FMCN) -- are implausible given the company's much smaller expenditure base. It further alleged that China Media is a "phantom company" that is conspicuously absent from local media coverage, industry reports, and analyst coverage, in contrast to names like Towona, Bus-Online, and VisionChina Media (Nasdaq: VISN).

Now what: Citron has been a vocal critic of other companies it claimed to be frauds, notably China-Biotics and, more recently, China Valves and Great Northern Iron, and many people take their analysis seriously. If I owned shares of a stock which Citron reported on, I'd want to examine their claims.

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At the time of publication, Ilan Moscovitz owned shares of Focus Media. Try any of our Foolish newsletter services free for 30 days. 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.