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 Internet portal SINA (Nasdaq: SINA) are sinking today, falling as much as 10% on heavy volume.

So what: There's a lot going on here: (1) SINA's fourth-quarter results were satisfying, but the outlook for the next quarter disappointed analysts; (2) one analyst firm upgraded the stock from "hold" to "buy," and another went in the opposite direction; (3) SINA is spending $66 million on a 19% stake in Chinese fashion retail portal Mecox Lane (Nasdaq: MCOX).

Now what: That's a lot of news to digest, but investors clearly focused more on the bad than the good. But is that fair? Conservative guidance doesn't mean much for a company in the habit of beating expectations, as SINA is, and Mecox Lane is a profitable cash machine with serious growth that should add to SINA's earnings right away. This looks like a knee-jerk overreaction and a potential buy-in point for this longtime Stock Advisor pick.

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