SMN2 and proximal spinal muscular atrophy: Being reminiscent of what occurs in SMA patients, who carry at least one or more copies of the SMN2 gene, these mice express, with high variability, motor deficit and spinal cord pathology, which is characterized by degenerating MNs and muscle denervation (Hsieh-Li et al. 2000; Monani et al. 2000b).