Updated

U.S. stock indexes posted significant gains on Monday, with the Dow climbing roughly 450 points and broad-based rallies across major sectors. However, underlying market breadth has deteriorated notably, with about 60% of S&P 500 constituents now trading more than 20% below their all-time highs—a pattern not observed since 1999. The weakness below the surface suggests that gains are concentrated in a narrow set of mega-cap names, primarily in technology and AI-related sectors.