Casimersen (Amondys 45™) is an antisense oligonucleotide of the phosphorodiamidate morpholino oligomer subclass developed by Sarepta Therapeutics for the treatment of Duchenne muscular dystrophy (DMD) in patients who have a mutation in the DMD gene that is amenable to exon 45 skipping. Administered by intravenous infusion, casimersen is designed to bind to exon 45 of the DMD gene pre-mRNA, resulting in skipping of this exon during mRNA processing, intended to allow for production of an internally truncated but functional dystrophin protein in patients with DMD. Casimersen received its first approval on 25 February 2021, in the USA, for the treatment of DMD in patients who have a confirmed mutation of the DMD gene that is amenable to exon 45 skipping. The approval, granted under the US FDA Accelerated Approval Program, was based on an observed increase in dystrophin production in skeletal muscle in patients treated with casimersen. Casimersen is continuing in phase III development for the treatment of DMD in several other countries worldwide. This article summarises the milestones in the development of casimersen leading to this first approval for DMD. As with other approvals under the Accelerated Approval Program, continued approval for this indication may be contingent upon verification of a clinical benefit in confirmatory trials.