The S&P 500 Index (SNPINDEX:^GSPC) continued its remarkable recent bull run, posting an eighth straight day of gains. More impressively, of course, is the nearly 150% pop the benchmark has enjoyed since the lows of 2009 when the financial crisis had the nation in a panic. The index added 2 points, or 0.1%, on Monday, closing at 1,682, another all-time record close. Unfortunately, the following three companies held back the index instead of helping it along to new highs.

Alexion Pharmaceuticals (NASDAQ:ALXN) posted the steepest losses of any stock in the 500-company group, shedding 5.5%. Monday's sell-off follows a wildly successful day for the stock on Friday, when shares jumped more than 12% on reports that the company was a potential takeover target for Roche. With some analyst pessimism today that Alexion may not really be primed for an immediate takeover, the stock understandably lost some major steam.

Residentail homebuilder PulteGroup (NYSE:PHM), whose shares are more than twice as volatile as the stock market itself, slumped 3.4%. Here we have another case of profit-taking after recent gains; the stock jumped more than 5% in two separate days of trading last week. Overall, today's losses are almost irrelevant to long-term investors, who must have been pleased with Federal Reserve Chairman Ben Bernanke's vow to keep up quantitative easing efforts last week.

Lastly, American Tower (NYSE:AMT) shares lost 2.2% Monday. The company, which operates as a real estate investment trust, owns and operates thousands of wireless communications real estate properties across the world. The stock was downgraded by Wall Street outfit ISI Group on the heels of AT&T's acquisition of Leap Wireless . Those two together accounted for about 19% of American Tower's operating revenues in the most recent quarter, so any change in the dynamics of the pricing structure for its antenna space could seriously impact the company.