It is caused by homozygous mutations in the survival motor neuron (SMN) 1 gene that leads to deficiency of SMN protein, which is predominant in the anterior horn cells.[1] Recently, Nusinersen, an antisense oligonucleotide that modulates pre-mRNA splicing to promote exon 7 inclusion of the SMN2 mRNA transcripts resulting in enhanced production of a functional SMN protein was approved as a disease-modifying therapy for SMA. The gene discussed is SMN1; the disease is proximal spinal muscular atrophy.