A revolutionary cancer therapy that uses genetically engineered immune cells has been approved by the U.S. Food and Drug Administration, ushering in a new era of cancer treatment.

The FDA calls the treatment, made by Novartis, the “first gene therapy” in the U.S. The therapy is designed to treat an often-lethal type of blood and bone marrow cancer that affects children and young adults. Known as a CAR-T therapy, the approach has shown remarkable results in patients. The one-time treatment will cost $475,000, but Novartis says there will be no charge if a patient doesn't respond to the therapy within a month.

“We’re entering a new frontier in medical innovation with the ability to reprogram a patient’s own cells to attack a deadly cancer,” said FDA commissioner Scott Gottlieb in a statement.

The therapy, which will be marketed as Kymriah, is a customized treatment that uses a patient’s own T cells, a type of immune cell. A patient’s T cells are extracted and cryogenically frozen so that they can be transported to Novartis’s manufacturing center in New Jersey. There, the cells are genetically altered to have a new gene that codes for a protein—called a chimeric antigen receptor, or CAR. This protein directs the T cells to target and kill leukemia cells with a specific antigen on their surface. The genetically modified cells are then infused back into the patient.

In a clinical trial of 63 children and young adults with a type of acute lymphoblastic leukemia, 83 percent of patients that received the CAR-T therapy had their cancers go into remission within three months. At six months, 89 percent of patients who received the therapy were still living, and at 12 months, 79 percent had survived.

An estimated 3,100 patients aged 20 and younger in the U.S. are diagnosed with acute lymphoblastic leukemia each year, making it the most common childhood cancer, according to the National Cancer Institute. Current treatment options include chemotherapy and stem-cell transplants, but about 600 pediatric and young adult patients with the disease relapse each year, and many remain incurable.

David Mitchell, founder of an advocacy group called Patients for Affordable Drugs, said in a statement that the $475,000 cost is “excessive” and claims the federal government spent $200 million in early research on CAR-T therapy before Novartis purchased rights to the treatment. The group recently met with the company to appeal for a “fair” price for its therapy. Previous estimates predicted a price tag between $600,000 to $750,000.

The historic approval bodes well for rival companies Kite Pharma and Juno Therapeutics, which are also developing CAR-T therapies. Kite Pharma, which is awaiting FDA approval for its CAR-T therapy to treat a form of blood cancer in adults, was this week bought out by Gilead in a deal worth $11.9 billion.

Though the Novartis therapy has shown extraordinary results in patients, questions remain about how the company will be able to manufacture personalized therapies quickly enough to get them to patients across the country. Novartis says it takes an average of 22 days to create the therapy, from the time a patient's cells are removed to when they are infused back into the patient. Kymriah will initially be available at 20 U.S. hospitals within a month, Novartis says. Eventually, 32 total sites will offer the therapy.

CAR-T therapy has also been known to cause potentially life-threatening side effects in some patients, including neurological problems and a reaction called cytokine release syndrome. Juno Therapeutics ended a CAR-T study earlier this year after patients died from cerebral edema, or swelling in the brain. No patients treated with the Novartis CAR-T therapy have died from that complication, according to the company.

The FDA defines gene therapy as a medicine that “introduces genetic material into a person’s DNA to replace faulty or missing genetic material” to treat a disease or medical condition. This is the first such therapy to be available in the U.S., according to the FDA. Two gene therapies for rare, inherited diseases have already been approved in Europe.