The Food and Drug Administration has approved a new treatment for cancer that includes genetic modification, the first gene therapy to be approved in the U.S. market, the agency said Wednesday.

Kymriah, a cell-based gene therapy, has been approved by the FDA for children and young adults under the age of 25 that have a form of acute lymphoblastic leukemia (ALL). The treatment is for those who have not responded to initial treatment, which occurs in an about 15-20 percent of patients, or those who have relapsed.

Kymriah is a customized treatment, meaning each dose contains the patient’s own T-cells (white blood cells known as lymphocyte). The T-cells are collected and sent to a manufacturing center where they are genetically modified to include a new gene with the protein CAR (chimeric antigen receptor). That specific protein instructs T-cells to target and kill leukemia cells. When the cells are modified they are put back into the patient’s body to get rid of cancer cells. The treatment is also known as CAR-T.

The FDA’s unprecedented approval opens up the agency’s expansion of human gene modification in the U.S. market.

“We’re entering a new frontier in medical innovation with the ability to reprogram a patient’s own cells to attack a deadly cancer,” FDA Commissioner Scott Gottlieb said in a statement. “New technologies such as gene and cell therapies hold out the potential to transform medicine and create an inflection point in our ability to treat and even cure many intractable illnesses.”

More than 3,000 patients aged 20 or younger are diagnosed with ALL every year, according to the National Cancer Institute. The disease is a cancer of the bone marrow and blood that causes the body to make abnormal lymphocytes. ALL is the most common childhood cancer in the country and can progress rapidly.

“As a breakthrough immunocellular therapy for children and young adults who desperately need new options, Kymriah truly embodies our mission to discover new ways to improve patient outcomes and the way cancer is treated,” said Bruno Strigini, CEO of Novartis, the company that developed Kymriah.

“Kymriah is a first-of-its-kind treatment approach that fills an important unmet need for children and young adults with this serious disease,” Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research (CBER) said in a statement. “Not only does Kymriah provide these patients with a new treatment option where very limited options existed, but a treatment option that has shown promising remission and survival rates in clinical trials.”

The clinical trial of Kymriah involved 63 child and young adult patients. The FDA said 83 percent of the patients were in remission within three months of the treatment.

However, the treatment could cause severe side effects, including a severe response by the immune system called cytokine release syndrome (CRS) and neurological events, which could both be life-threatening. Other side effects include serious infections, low blood pressure (hypotension), acute kidney injury, fever, and decreased oxygen (hypoxia).

Novartis is required to conduct a post-marketing trial involving patients treated with Kymriah in order to evaluate long-term safety, the FDA said.

As for the price, Kymriah will cost $475,000, while those who don’t respond to the treatment within a month will not be obligated to pay.