Although, mutations in the SMN1 but not SMN2 are the cause of SMA, the expression of SMN2 is connected to the clinical phenotype of SMA, where patients with less severe types of SMA have more copies of the SMN2 gene than do patients with the more severe types of the disease (Niba et al. 2021). This evidence concerns the gene SMN1 and proximal spinal muscular atrophy.