The 5 biggest U.S. stocks account for almost 18% of the S&P 500's market value

The five biggest U.S. stocks — Apple, Microsoft, Alphabet, Amazon and Facebook — have grown so explosively that they account for nearly 18% of the S&P 500 index by market value, AP reports.

Why it matters: Never before have five companies held such powerful sway over the index, according to Morgan Stanley strategists.

The history: The last time five stocks controlled this much of the S&P 500 was during the tech bubble at the turn of the millennium.

• That bubble eventually popped, and stocks like Cisco and GE shrank to become smaller players. Microsoft was also among them, but it has since climbed back to the top.

One difference between this time and the dot-com bubble is that many analysts don't see prices as grossly over-inflated now.

• Each of today's Big Five is producing strong growth even when the global economy has been stuck in a sluggish pace for years.

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