The Nasdaq-100 rose 3.3% in September even as most of the U.S. stock market declined. The Russell 2000 fell 5.3%, the Dow dropped 4.1%, nine of 11 large-cap sectors ended lower, and 78% of S&P 500 stocks lost ground. Technology gained 4.5% and communication services rose 4.3%, the only two large-cap sectors to finish meaningfully higher. Their strength, particularly among AI-linked and mega-cap companies, was sufficient to lift the Nasdaq-100, which is heavily weighted toward large technology and growth stocks. The divergence shows how major market indexes are increasingly being driven by a relatively small group of companies. At the same time, the 10-year Treasury yield jumped 54 basis points in September to 5.29%, its largest monthly increase in four years. Nasdaq attributed the move to persistent inflation, higher energy prices, tighter monetary policy, and heavy government and corporate debt issuance. Higher borrowing costs generally weigh more heavily on smaller companies because they depend more on external financing and often face less favorable borrowing terms. The Russell 2000 consequently recorded its worst month since March 2025, while large-cap growth stocks remained comparatively resilient.