Chronic In Vivo CRISPR-Cas Genome Editing: Challenges, Long-Term Safety, and Outlook.

CRISPR/Cas systems have transformed molecular medicine, yet the field still lacks principled guidance on when transient editing suffices versus when sustained exposure through in vivo viral delivery is necessary and how to keep prolonged exposure safe. Notably, EDIT-101 was designed for a permanent edit in post-mitotic photoreceptors with lifelong Cas9 persistence. This review addresses this gap by defining the biological and therapeutic conditions that drive benefit from extended Cas activity while minimizing risk. We will (i) examine relationships between expression window and efficacy across Cas9/Cas12/Cas13 modalities, (ii) identify genome-wide off-target liabilities alongside orthogonal assays, and (iii) discuss controllable, self-limiting, and recallable editor platforms. By separating durable edits from persistent nuclease exposure, and by providing validated control levers, this work establishes a generalizable framework for safe, higher-efficacy CRISPR medicines. Furthermore, we highlight key studies in cell lines, murine models, non-human primates, and humans that examine the long-term effects of sustained expression of CRISPR/Cas systems and discuss the safety and efficacy of such approaches. Current evidence demonstrates promising therapeutic outcomes with manageable safety profiles, although there is a need for continued monitoring as CRISPR/Cas therapies are increasingly applied in clinical contexts and therapies are developed for broader clinical applications.