The first cancer treatment that involves reprogramming a patient’s own blood cells to fight cancer has been approved by the US Food and Drug Administration, leading the way for federal approval of other, similar efforts.

Kymriah is manufactured by the pharmaceutical company Novartis AG to treat children with acute lymphoblastic leukemia (ALL). It’s shown very encouraging results in clinical trials, but the price tag will be hefty: Analysts say it will cost “a fortune,” or maybe $700,000 for one course of treatment.

Kymriah is one type of so-called CAR-T cancer therapies. First, doctors take the patient’s white blood cells, or T cells, out of the body and add a special receptor called a chimeric antigen receptor (CAR). The receptor gives the T cells the ability to attack cancer cells. Then, these engineered cells are put back into the body. It’s a highly personalized form of medicine, since each dose must be tailored to the patient.

Trials have shown promising results: In one trial of 63 people with few remaining treatment options, 83 percent of participants using Kymriah were free of cancer after three months. But some have relapsed, and we don’t know the long-term effects. Plus, there can be serious side effects, such as inflammatory problems that have the potential to be fatal.

The demand for Kymriah may not be very high. It only treats ALL patients up to age 25 who have relapsed. And though there are more than 3,000 new cases of ALL in the US each year, most of these cases can be treated with standard therapies, according to STAT News.

Still, the approval marks a big step for companies working on other CAR-T gene therapies. Novartis is now developing Kymirah for other blood cancers, while Gilead Sciences’ Kite Pharma and Juno Therapeutics are doing the same thing.