SMN1 and proximal spinal muscular atrophy: The pathogenesis and severity of SMA relies on the type of mutations in the SMN1 gene and the homologous SMN2. The SMN2 isoform is rather unstable through the exclusion of exon7 and cannot fully compensate SMN1. However, increased SMN2 protein levels can render SMA to its milder courses (see for review Kolb and Kissel, 2015).