The U.S. Food and Drug Administration placed formal clinical holds on two Phase 3 trials of Intellia Therapeutics’ CRISPR candidate nexiguran ziclumeran (nex‑z) following a grade‑4 liver adverse event that hospitalized a participant. Intellia had already paused dosing voluntarily; the FDA said it will issue a formal letter within 30 days outlining deficiencies and requests for additional information. Intellia’s programs target transthyretin amyloidosis (ATTR) and had enrolled hundreds of patients across the two studies prior to the pause. The agency’s formal hold raises the level of regulatory scrutiny and will likely delay readouts and potential commercialization timelines, analysts said in coverage cited by MedCity News and other outlets. The development underscores heightened regulatory caution around in vivo gene editing: sponsors will need to work closely with regulators to define new safety monitoring and mitigation strategies before restart is authorized.