All patients with SMA have an insufficient amount of a survival motor neuron (SMN) protein, which is encoded by two highly homogenous genes, survival of motor neuron 1 (SMN1) and its copy gene, SMN2. SMN2 is differentiated from SMN1 by one single nucleotide variant (C→T) in exon 7. Here, SMN2 is linked to proximal spinal muscular atrophy.