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A drug for Angelman syndrome failed its final trial. Researchers say the science is not done.

9/5/2026

A late-stage trial for an experimental Angelman syndrome drug came back negative this week, dealing a major blow to patients and to the company behind it.

Late-stage, in clinical testing, means the final round of human trials a medicine must complete before the maker can ask regulators to approve it for public use.

Ultragenyx, which developed the drug, announced the outcome. Angelman syndrome is a rare genetic disease.

It causes severe developmental delays, and families living with it have had few treatment options available to them. The trial that just failed was testing whether a genetic medicine could change that.

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