U.S. stocks closed sharply lower Friday, with the Dow Jones Industrial Average dropping 100 points, as market professionals weighed the ongoing Greek debt crisis and a disappointing report showing the economy shrank in the first quarter. Economists are looking ahead to the June 5 release of the jobs report for May, after U.S. employers rebounded and created more jobs in April following a sharp slowdown in March. 

The Dow (INDEXDJX:.DJI) dropped 99.23 points, or 0.55 percent, to close at 18,027. The Standard & Poor's 500 (INDEXNASDAQ:.IXIC) dipped 11.87 points, or 0.56 percent, to end at 2,109. The Nasdaq composite (INDEXSP:.INX) fell 23.66 points, or 0.46 percent, to finish at 5,074.

The U.S. employment report released next Friday is expected to show employers added 225,000 jobs in May, up from 223,000 in April, according to analysts polled by Thomson Reuters. The unemployment rate is expected to hold steady at 5.4 percent.

The Dow tumbled more than 150 points after the open Friday amid a report that showed the U.S. economy shrank in the first three months of the year, driven by a slowdown in shipping due to the West Coast port disputes. Market professionals also remained cautious as concerns grow over the Greek debt crisis.

Another factor behind the market's dull performance: the old adage “sell in May and go away,” a historic seasonal trading pattern that refers to the tendency of the stock market to post its best returns between November and April and record weaker performance between May and October.

For the month, the Dow gained 172 points, or 0.96 percent, and the S&P 500 index rose 22 points, or 1.05 percent. The Nasdaq gained 129 points, or 2.6 percent, in May.

The Dow Jones Transportation Average dropped nearly 1 percent Friday after tumbling below its 200-day moving average Tuesday. Friday's losses were led by a 3 percent decline from freight transportation and logistics company Con-way Inc. (NYSE:CNW).

“The railroads are under pressure because they’re not transporting as much petroleum products,” said Karl Snyder, chief market strategist at Garden State Securities. “Shipments are down and that is having a negative effect on their revenues.”

General Electric Company (NYSE:GE) and Visa Inc. (NYSE:V) led the Dow lower Friday, losing 1.3 percent and 1.2 percent, respectively.

Meanwhile, healthcare company Merck & Co. Inc. (NYSE:MRK) was the biggest gainer in the blue-chip index, adding 2 percent.

Shares of Humana Inc. (NYSE:HUM) soared 20 percent Friday to close at $214.65 amid reports the health insurer is considering a sale of the company, the Wall Street Journal reported. Competitors Aetna Inc. (NYSE:AET) and Cigna Corporation (NYSE:CI) are among bidders that have held preliminary discussions with the company, the Wall Street Journal said, citing people familiar with the matter.

Dow component Intel Corporation (NASDAQ:INTC) added 1.3 percent amid reports the chipmaker is close to a deal to buy smaller chipmaker Altera Corporation (NASDAQ:ALTR) for about $15 billion, according to a report in the New York Post. Altera’s stock price jumped 4 percent.