All the three SMA types are caused by homozygous mutations of the survival motor neuron 1 (SMN1) gene (5q13), which in about 95% of cases is represented by the functional absence of this gene due to deletion or its conversion to SMN2. This latter gene, mapped within the SMA critical region, is not directly related to the disease, but is considered a disease-modifying gene because its copy number relates to the disease severity and survival of affected patients [39–42]. Here, SMN2 is linked to proximal spinal muscular atrophy.