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 young-adult apparel retailer Express (NYSE:EXPR) sank 10% today after issuing guidance that disappointed Wall Street.

So what: The company's fourth-quarter results managed to top expectations -- EPS of $0.75 on revenue of $728.7 million versus the consensus of $0.74 and $722.5 million -- but downbeat guidance for the full year is forcing analysts to recalibrate their growth estimates. In addition, gross margin during the quarter declined 210 basis points to 35.1%, reigniting concerns over increasing costs and competition.

Now what: Management now sees full-year EPS of $1.40-$1.54, well short of Wall Street's estimate of $1.72. "While our guidance anticipates a softer start to the year, reflecting the impact of reduced traffic levels and consumer spending in the month of February, our spring merchandise has been received favorably by our customers, resulting in an improvement in conversion," said Chairman and CEO Michael Weiss. Of course, with the stock still up more than 60% from its 52-week lows even after today's plunge, I'd wait for more of a pullback before buying into that optimism.

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