0559 GMT - Broad index and retirement funds are now tied to the AI boom, with technology stocks accounting for about 40% of the S&P 500. Only three chip makers account for more than 25% of the MSCI EM index, says Ipek Ozkardeskaya, senior analyst at Swissquote. AI is the "cornerstone that must not crack," she adds. For now, U.S. equities will have the seasonal wind at their backs and the equity rally could extend into the year-end, she says. The worst-case scenario would be that something cracks in the AI story, triggering a notable pullback, she adds.