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The S&P 500's recent push back toward record territory has been driven by an unusually small number of stocks, with market breadth now at its narrowest since the dot-com peak. Analysts note this concentration raises questions about the sustainability of the rally if leadership doesn't broaden. The trend underscores growing reliance on a handful of mega-cap, often AI-linked companies to power index gains, a dynamic that has drawn comparisons to prior periods of narrow market leadership that preceded downturns.