SMN1 and proximal spinal muscular atrophy: The proximal form of SMA is mainly due to deletions or mutations in the telomeric copy of the survival motor neuron gene-1 (SMN1) [2] SMA III is a milder form of SMA starting after the achievement of walking and is subdivided in SMA IIIa with an onset before 3 years and SMA IIIb after 3 years.