Due to its defective splicing pattern, the highly homologous copy gene SMN2 generates an insufficient amount of functional SMN protein: therefore, the SMN2 copy number inversely correlates with the severity of SMA phenotype (Lorson et al., 2010), accounting for the presence of four main clinical SMA types (SMA I-IV), characterized by different age of onset and disease progression (Lefebvre et al., 1995). Here, SMN1 is linked to proximal spinal muscular atrophy.