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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