S&P SmallCap 600 in Focus: BKUNA PHTN

The S&P 600--which tracks stocks with a market capitalization between $300 million and $2 billion--declined 6.04 points, or 1.6 percent, to 372.92.

Small-cap companies make up about 3 percent of the total U.S. securities market, but are often cited as good investments due to their low valuations and potential to grow into big-cap stocks.

BankUnited Financial Corp. plunged 26 cents, or 19.1 percent, to $1.10 after touching an all-time low of $1.05 earlier in the session. Shares of the Florida-based regional bank have withered more than 80 percent since the start of the year.

Auto parts maker Standard Motor Products Inc. slumped $1.21, or 12.6 percent, to $8.39.

Select Comfort Corp. tumbled 24 cents, or 10.9 percent, to $1.96. Shares of the mattress maker fell as low as $1.80 in intraday trading, a price not seen since 2002.

Leading the gainers, Photon Dynamics Inc. surged $3.60, or 31.1 percent, to $15.16 after Israel's Orbotech Ltd. said it would buy the maker of test equipment for LCD displays for $15.60 per share, a 35 percent premium over Photon's Wednesday closing price.

The stock hit a 52-week high of $15.28 earlier in the session.

Parexel International Corp. jumped $1.87, or 7.4 percent, to $26.92 after analysts said the pharmaceutical services company will enjoy strong profit and revenue growth from demand for its clinical research services. A Needham analyst upgraded the company to "Outperform" from "Market Perform" in a note to investors.

Shares of over-the-counter drug maker Chattem Inc. rose $3.23, or 5.1 percent, to $66.55.