Monday has been a big day of worry for the stock market so far this year: The S&P 500 has fallen during every single Monday session in 2013. This morning, the market seemed likely to continue that run of the Monday blues, as government action from China to try to avoid a real-estate bubble raised concerns about the emerging economy's growth. Moreover, ongoing worries about the U.S. government budget crisis appear likely to linger for some time. In response, the Dow Jones Industrials (DJINDICES:^DJI) are down 34 points as of 10:50 a.m. EST, with broader markets also experiencing moderate losses.

Within the Dow, Caterpillar (NYSE:CAT) and Boeing (NYSE:BA) are among the hardest-hit. Caterpillar, which is down 1.6%, is particularly sensitive to news about China, as its worldwide corporate strategy puts a lot of focus on taking advantage of the fast rate of economic growth there. For Boeing, which is down 1.2%, the fact that the company made a substantial sale worth about $4.8 billion to Air China underscores the potential growth available in the Chinese aerospace industry. However, Boeing's first priority right now should be to get its 787 Dreamliner back in the air after safety concerns forced regulators to ground the new model.

Elsewhere, Berkshire Hathaway (NYSE:BRK-B) has fallen 1.2% after releasing its annual letter to shareholders last Friday. Despite a strong gain, Berkshire's book-value growth lagged behind the S&P 500's return, prompting CEO Warren Buffett to say that 2012 was a "subpar" year for the company. Yet promising performance from potential Buffett successors within the company provides hope for Berkshire's future even once Buffett is no longer at the helm.

Finally, 3-D printing company Stratasys (NASDAQ:SSYS) has climbed more than 7% after announcing favorable earnings. Stratasys beat estimates, but more importantly, the company gave positive guidance for 2013. With recent share-price declines reflecting investors' concerns that the stock had gotten ahead of itself, CEO David Reis cited Stratasys' recent merger with peer Objet as creating new opportunities for the combined companies going forward.