GOLDMAN SACHS: Stocks have never been more vulnerable to the failure of a few mega-companies — and the risks of a blunder are quickly piling up

· Five technology stocks now hold a record 22% share of the S&P 500 and have contributed the most to its gains this year. 
· Their outsized role has made the market more vulnerable than ever to a single company's slip-up, says David Kostin, the chief US equity strategist at Goldman Sachs. 
· He flagged multiple risks that...