The Food and Drug Administration on Friday approved the first gene therapy for a type of spinal muscular atrophy, a lifesaving treatment for infants that will also be the most expensive drug in the world.

Known as Zolgensma, the gene therapy treats children under 2 years of age with spinal muscular atrophy, an inherited neuromuscular disease that causes progressive loss of muscle function. The most severe form of SMA causes infants to die or rely on permanent breathing support by the age of 2. The disease is caused by a defect in a gene that makes SMN, a protein necessary for the survival of motor neurons. Zolgensma uses a re-engineered virus to deliver a functional copy of the defective gene so that SMN protein can be produced.