enGene’s Phase II LEGEND interim updates for detalimogene voraplasmid delivered a deterioration versus earlier expectations, triggering a sharp investor selloff. The updated data showed complete response rates falling from prior readouts, with durability also looking weaker at later timepoints. The stock moved aggressively as investors digested results for BCG-unresponsive NMIBC with carcinoma in situ (CIS±papillary), underscoring how quickly market sentiment can turn in crowded bladder cancer immuno-gene competition. enGene signaled it plans to engage with the FDA and continue longer-term durability evaluation. For clinical and regulatory observers, the event highlights how milestone-based durability signals—rather than early efficacy—are increasingly shaping next steps for gene therapy candidates in real time.