The treatment uses a single infusion to insert the genes people with hemophilia A are missing, and may work for years, according to data presented to regulators. It’s also the latest FDA-approved gene therapy, heralding what some scientists say is the future of treating thousands of diseases.

The FDA has approved gene therapies for conditions including sickle cell anemia and spinal muscular atrophy. Use of the technology is relatively rare, in part because gene therapies are among the most expensive drugs in the world. Roctavian will cost $2.9 million for a single infusion; Hemgenix, which treats another form of hemophilia, costs $3.5 million per use, according to Reuters. Still, scientists say the technology will be a crucial part of 21st century medicine.

Here’s what you need to know.