Shares in the world's largest commercial real estate broker, CB Richard Ellis Group Inc., tumbled 6 percent Tuesday after the company was downgraded ahead of its first quarter earnings.

Citi Investment Research analyst Patrick Burton downgraded the shares saying the Los Angeles-based company is overpriced given the sagging commercial real estate market.

"We do not expect that the commercial real estate market can turn around until the late 2009 to 2010 timeframe," wrote Burton in the research report.

The firm reports its first quarter earnings after the market closes today. Analysts polled by Thomson Financial expect a profit of 21 cents per share on revenues of $1.15 billion.

The shares have gained more than 50 percent since hitting a 52-week low of $15.23 in January. But they closed down 6 percent to $22.17 in early trading Tuesday on the New York Stock Exchange.

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