Stock indexes fell again, capping a week in which the Dow Jones Industrial Average posted its steepest decline since December, driven by large share-price swings at widely followed companies issuing quarterly earnings reports.

Shares of biotechnology companies led Friday’s decline, after Biogen plunged 22% on a soft sales outlook. The biotech pullback countered a 9.8% gain in Amazon.com, the latest stock-price surge in a technology company that posted better-than-expected earnings.

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