SMN1 and proximal spinal muscular atrophy: Three approved therapies for the treatment of SMA are currently available: nusinersen, a 2-O-methoxyethyl phosphorothioate-modified antisense oligonucleotide that modifies the splicing of the SMN2 pre-mRNA, increasing the production of full-length SMN protein from the SMN2 gene; onasemnogene abeparvovec (AVXS-101), a gene replacement therapy; and risdiplam, an orally administered small-molecule SMN2 splicing modifier that was recently approved by the Food and Drug Administration for the treatment of SMA in adults and children (2 months of age and older).