Spinal muscular atrophy (SMA) is a rare autosomal recessive disorder 
characterized by muscle atrophy and weakness resulting from motor neuron 
degeneration in the spinal cord and brainstem. It is most commonly caused by 
insufficient levels of survival motor neuron (SMN) protein (which is critical 
for motor neuron maintenance) secondary to deletions or mutations in the SMN1 
gene. Nusinersen (SPINRAZA™) is a modified antisense oligonucleotide that binds 
to a specific sequence in the intron, downstream of exon 7 on the pre-messenger 
ribonucleic acid (pre-mRNA) of the SMN2 gene. This modulates the splicing of the 
SMN2 mRNA transcript to include exon 7, thereby increasing the production of 
full-length SMN protein. Nusinersen is approved in the USA for intrathecal use 
in paediatric and adult patients with SMA. Regulatory assessments for nusinersen 
as a treatment for SMA are underway in the EU and several other countries. This 
article summarizes the milestones in the development of nusinersen leading to 
this first approval for SMA in paediatric and adult patients.