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Clinical trial · Sickle cell disease
CLIMB SCD-121 — the trial that led to the first CRISPR medicine approval
Official title: A Safety and Efficacy Study Evaluating CTX001 in Subjects With Severe Sickle Cell Disease
CompletedPhase II/IIINCT03745287
Clinical researchBeing tested in people in registered clinical trials. Being in trials is not evidence that a treatment works or is safe.
What this trial is testing
This is the study that made Casgevy an approved medicine. Participants with severe sickle cell disease had their own blood stem cells edited and returned. The main question was whether they would stop having the severe pain crises that define the disease — and for the large majority, they did.
An open-label, single-arm, multi-site Phase 2/3 study of autologous CRISPR-Cas9-modified CD34+ cells (CTX001, later exagamglogene autotemcel) in patients aged 12–35 with severe sickle cell disease. The primary endpoint was freedom from severe vaso-occlusive crises for at least 12 consecutive months. Results supported approvals by the MHRA, FDA and EMA.
What a trial result does and does not mean
Early-phase results come from small numbers of participants and short follow-up. They can change with more people and more time, and a trial that is recruiting, active or even completed has not established that a treatment works. Enrolment criteria are strict; eligibility is decided by the trial site, never by a website.
Written by The CRISPR Atlas editorial teamLast updated Aug 19, 2026
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