Exondys 51 is not technically gene therapy, but it does treat people with Duchenne muscular dystrophy who have a confirmed genetic mutation in their dystrophin gene which means they are unable to make the protein dystrophin. Dystrophin is needed for muscle repair. Without it, muscle cells become damaged and weakened over time, which can affect voluntary movement, such as walking.

Gene therapy technically refers to any technique that modifies a person's genes to treat or cure disease. The therapy may involve replacing a disease-causing gene with a healthy copy or inactivating a disease-causing gene that does not work properly. Exondys 51 does not work like this. Instead, it skips exon 51 (exons are sequences of DNA present in mature messenger RNA, some of which encode for specific proteins), which allows the body to make a shorter form of the dystrophin protein which still has some function. Exondys 51 is not a cure for muscular dystrophy but may lessen muscle weakness and muscle wasting. But it can only be used by children with a confirmed genetic mutation in their dystrophin gene that responds to skipping exon 51.

Exondys 51 belongs to the class of medicines known as antisense oligonucleotides. These help cells skip over a specific exon during splicing, which allows them to join a different set of exons together to produce a protein that is shorter than usual but may have some function.