DeepSeek's emergence is shaking up investor confidence of the AI story that has been lifting the U.S. bull market in the past two years.

Nasdaq posted its biggest one-day percentage drop since Dec. 18 on Monday as a low-cost Chinese artificial intelligence model prompted a steep selloff in U.S. chipmakers.

The Nasdaq & S&P 500 fell after Chinese startup DeepSeek shows AI can be built cheaply, sparking fears AI spending will stall. The blue-chip Dow rose.