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In February 2026, the US Food and Drug Administration (FDA) published a draft guidance on a new plausible mechanism framework for the development and approval of individualized therapies for genetic conditions. Here, we report initial proof-of-concept studies supporting a customizable prime editing platform geared to the treatment of 7 urea cycle disorders (UCDs) and other liver-centered disorders, as well as the outcome of a formal meeting with the FDA to discuss the use of the platform in an "umbrella-of-umbrellas" clinical trial including subjects with any of the 7 UCDs. We anticipate our findings will be of interest to academic investigators and industry sponsors who wish to pursue expeditious FDA approvals of therapies for ultra-rare diseases using the plausible mechanism framework.