Spinal muscular atrophy (SMA) is a disease caused by a mutation in the survival motor neuron 1 (SMN1) gene and babies afflicted by it rarely live beyond their second birthday. 

Children with this condition have problems holding their head up, swallowing and breathing. These symptoms may be present at birth or may present by the age of 6 months.

SMA caused by mutations in the SMN1 gene is generally classified into several subtypes, based on the age of onset and severity; infantile-onset SMA is the most severe and most common subtype. 

Onasemnogene abeparvovec is an SMN-enhancing therapy. It works by replacing the missing or mutated SMN1 gene through a virus named AAV9  (Adeno Associated Virus) which carries the replacement gene into the body.

It is given through an intravenous (IV) infusion that takes about an hour. It is a one-time treatment