SMN1 and proximal spinal muscular atrophy: Spinal muscular atrophy (SMA) is a neurodegenerative disorder caused by mutations in SMN1 that determine a reduction in SMN protein [1,2] and a resulting loss of alpha motor neurons (MNs) in the brainstem and spinal cord, followed by progressive muscle weakness and atrophy, as well as early death [1,2].