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Two patients in a study of a high-dose gene therapy for a rare muscle disorder have died, heightening worries about the safety of potent treatments under development for other diseases.

Audentes Therapeutics, a San Francisco-based unit of the Japanese drug firm Astellas, told patient groups in a June 23 letter, a copy of which was obtained by STAT, that the two patients died after having serious liver problems that appeared related to the company’s experimental treatment for X-linked myotubular myopathy, a condition that causes severe muscle weakness. Most patients with the disorder, also known as XLMTM, do not survive childhood. In previous data from the study, Audentes has said its treatment improved patients’ ability to breathe, with some getting off of ventilators.

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