To the Editor: Frangoul and colleagues (Jan. 21 issue)1 report striking results with gene editing for sickle cell disease and transfusion-dependent ¦Â-thalassemia with the use of the BCL11A-targeted CRISPR (clustered regularly interspaced short palindromic repeats)¨CCas9 nuclease system. They describe two groundbreaking clinical cases that provide compelling proof-of-concept evidence that CRISPR-Cas9¨Cbased targeted gene editing can be successfully achieved in human hematopoietic stem and progenitor cells. These cases also suggest a substantial prospect of meaningful therapeutic effects. To draw as much information as possible from these case histories, it would be important to know whether the investigators followed individual clonal gene-editing events . . .
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