Maria Aspen

These companies dominate the Nasdaq and the broader S&P 500 index, meaning that if tech stocks are having a great run, so is the overall market.

But the opposite also holds true.

And some investors are increasingly worried about what happens if the tech bubble pops.

On Tuesday, the CEOs of Goldman Sachs and Morgan Stanley both urged investors to be ready for an eventual market sell-off.

Hours later, the Nasdaq closed down more than 2%.

Maria Aspin, NPR News, New York.