Casgevy is a one-time gene therapy that uses CRISPR gene editing to reactivate fetal hemoglobin, a type of hemoglobin your body naturally produced before birth but largely switched off in infancy. By restoring production of this protein, Casgevy treats the root cause of sickle cell disease and transfusion-dependent beta-thalassemia rather than managing symptoms. It is approved for patients 12 years and older, and the entire treatment process takes roughly nine months to a year from start to finish.
The Gene Target Behind the Treatment
Everyone is born making fetal hemoglobin, which carries oxygen effectively and doesn’t form the rigid, sickle-shaped red blood cells that cause pain crises in sickle cell disease. Within the first year of life, a gene called BCL11A flips on and acts as a master switch, silencing fetal hemoglobin production and telling your body to make adult hemoglobin instead. In healthy individuals, adult hemoglobin works perfectly fine. In people with sickle cell disease or beta-thalassemia, the adult hemoglobin is defective.
Casgevy doesn’t fix the defective adult hemoglobin gene. Instead, it uses a different strategy: it disables the switch that turned off the good stuff. Specifically, CRISPR-Cas9 (a molecular tool that cuts DNA at a precise location) targets an enhancer region inside the BCL11A gene. This enhancer is what allows BCL11A to function in red blood cells. When that enhancer is disrupted, BCL11A can no longer block fetal hemoglobin production. The result is that your red blood cells start churning out fetal hemoglobin again, compensating for the faulty adult version.
What makes this target especially clever is that the enhancer is specific to red blood cells. BCL11A does other jobs in other cell types, like helping with brain development, but editing this particular enhancer only affects its activity in the blood-forming cells. That selectivity is what makes the treatment viable.
Step by Step: What the Treatment Looks Like
Casgevy isn’t a single injection. It’s a multi-stage process that requires significant time, hospitalization, and preparation. Here’s what each phase involves.
Preparation and Stem Cell Collection
About two months before the collection procedure, you begin receiving regular red blood cell transfusions to stabilize your condition. When it’s time to collect stem cells, you’re admitted to the hospital and a catheter is placed in a vein under sedation. You then receive a medication that coaxes blood-forming stem cells out of your bone marrow and into your bloodstream, a process called mobilization. Those stem cells are filtered out of your blood through apheresis, which works somewhat like dialysis. Collection typically takes two to three days, and sometimes additional rounds are needed to gather enough cells.
Manufacturing the Edited Cells
Once collected, your stem cells are shipped to a specialized lab where the actual CRISPR editing happens. Technicians introduce the Cas9 protein along with a guide RNA that directs it to the BCL11A enhancer. The tool makes a precise cut in the DNA, and when the cell repairs the break, the enhancer is disrupted. This manufacturing process takes several months. During this waiting period, you go home and continue your usual medical care.
Conditioning Chemotherapy
When the edited cells are ready, you return to the hospital for what may be the hardest part of the process. Before the new cells can be infused, the existing stem cells in your bone marrow need to be cleared out to make room. This is done with a powerful chemotherapy drug that wipes out your current bone marrow, a step called myeloablative conditioning. The conditioning phase lasts several days and causes significant side effects: very low blood cell counts, mouth sores, nausea, vomiting, abdominal pain, headache, fever from low immune cell counts, and itching. These side effects are consistent with what patients experience during any bone marrow transplant and are caused by the chemotherapy, not by the gene-edited cells themselves.
Infusion and Engraftment
About a week after conditioning, you receive the edited stem cells as a single intravenous infusion. The cells travel to your bone marrow and begin to engraft, meaning they settle in and start producing new blood cells. This recovery phase requires close monitoring. Hospital stays average about one month, though some patients stay four to six weeks. During this time, your immune system is severely weakened, so the medical team watches closely for infections and other complications. Once your blood counts recover and you’re stable enough to go home, follow-up appointments continue for 15 years.
How Well It Works
The clinical trial results for Casgevy have been striking for both conditions it treats. In the sickle cell disease trial (CLIMB SCD-121), which enrolled patients aged 12 to 35 who had experienced at least two severe pain crises per year, 29 out of 30 evaluable patients (96.7%) were completely free of vaso-occlusive crises for at least 12 consecutive months after treatment. For people accustomed to repeated emergency room visits and hospitalizations from pain episodes, that represents a dramatic shift.
For transfusion-dependent beta-thalassemia, the results were similarly strong. Of 42 patients evaluated, 39 (about 93%) achieved transfusion independence, maintaining hemoglobin levels at or above 9 g/dL without any red blood cell transfusions for at least 12 consecutive months. These are patients who previously required transfusions every few weeks to survive.
Safety and the Question of Precision
The most closely watched safety concern with any CRISPR therapy is off-target editing, meaning the molecular scissors cut somewhere in the genome they weren’t supposed to. The FDA identified this as the major risk of Casgevy during its review. In testing, researchers looked for unintended edits across a range of candidate sites in the genome using multiple analytical methods. No off-target editing was detected in the cells tested.
That said, the FDA noted important limitations. Only some genetic variants that could potentially harbor off-target cuts were empirically tested, partly due to sample limitations. The diversity of the patient population means there could be rare genetic variants where off-target edits occur that haven’t been checked yet. As a condition of approval, Vertex Pharmaceuticals (the maker of Casgevy) is required to conduct a postmarketing bioinformatics study to assess off-target risks across a more diverse population, along with a 15-year observational safety study specifically monitoring for secondary cancers. The conditioning chemotherapy itself also carries a theoretical risk of contributing to blood cancers over the long term, a concern that applies to all bone marrow transplant procedures.
Cost and Access
Casgevy carries a list price of approximately $2.2 million for a one-time treatment. Major insurers, including Aetna, have established medical policies covering it when clinical criteria are met, treating it as medically necessary for eligible patients with sickle cell disease or transfusion-dependent beta-thalassemia. Medicare also has coverage pathways. The practical reality, though, is that access depends on being treated at a specialized center equipped to handle the cell collection, conditioning chemotherapy, and post-infusion monitoring. Only a limited number of medical centers currently offer the treatment, which can create geographic barriers and long wait times.
Who Is Eligible
The FDA approved Casgevy for patients 12 years and older with two specific conditions: sickle cell disease with recurrent vaso-occlusive crises, and transfusion-dependent beta-thalassemia. For sickle cell disease, “recurrent” generally means a documented history of frequent pain crises severe enough to require medical attention. For beta-thalassemia, you need to have been dependent on regular transfusions. Patients must also be healthy enough to tolerate the myeloablative conditioning chemotherapy, which rules out some individuals with significant organ damage or other complicating conditions. Your hematologist and the transplant team together determine whether the benefits outweigh the risks in your specific case.