Currently, Nvidia accounts for nearly 8 percent of the S&P 500. That’s the highest weighting for a single stock in 45 years, according to Todd Sohn, senior ETF strategist at Strategas Securities. For context, industrials account for 8.7 percent of the index, while healthcare stands at 9.1 percent.

The S&P 500 was virtually unchanged in midday trading and just a bit below its all-time high, even as more than four out of every five stocks within the index fell.