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CRISPR-Cas12a Gene Editing of HBG1 and HBG2 Promoters to Treat Sickle Cell Disease. BACKGROUND: Renizgamglogene autogedtemcel (reni-cel) is an investigational clustered regularly interspaced short palindromic repeats (CRISPR)-Cas12a gene-edited autologous hematopoietic stem-cell therapy. The therapy was designed to disrupt the BCL11A binding sites in the HBG1 and HBG2 promoters to reactivate fetal hemoglobin production for the treatment of sickle cell disease. METHODS: We conducted a phase 1-2, multicenter, open-label, single-group study involving patients with severe sickle cell disease who were 12 to 50 years of age and had had at least two severe vaso-occlusive events per year in the previous 2 years. The patients received a single infusion of reni-cel after myeloablative conditioning with busulfan. The patients were monitored for engraftment, hemoglobin-related measures, allelic editing levels, vaso-occlusive events, and adverse events over a 24-month period. The study was terminated early on the basis of the sponsor's reassessment of clinical development priorities. Results of an analysis that was not prespecified are reported. RESULTS: As of October 29, 2024, a total of 28 patients with severe sickle cell disease had been treated with reni-cel. The median duration of follow-up was 9.5 months (range, 0.7 to 25.2). Among 27 patients who had neutrophil and platelet engraftment by the data-cutoff date, neutrophil engraftment occurred after a median of 23 days (range, 14 to 29), and platelet engraftment occurred after a median of 25 days (range, 17 to 51). At month 6, among 18 patients with at least 6 months of available data, the mean (±SD) total hemoglobin level (9.8±1.7 g per deciliter at baseline) had increased to 13.8±1.9 g per deciliter, and the mean percentage of fetal hemoglobin (2.5±2.5% at baseline) had increased to 48.1±3.2%; both measures were maintained at or above these values thereafter. One patient had two severe vaso-occlusive events after infusion. Adverse events were consistent with those that occur after myeloablative busulfan-based conditioning and autologous hematopoietic stem-cell transplantation. CONCLUSIONS: Treatment with reni-cel led to normalization of the total hemoglobin level and an increase in the percentage of fetal hemoglobin, with no vaso-occlusive events occurring in 27 of 28 patients after infusion. These results support further investigation of this gene-editing approach in the treatment of severe sickle cell disease. (Funded by Editas Medicine; RUBY ClinicalTrials.gov number, NCT04853576.).