SMN1 and proximal spinal muscular atrophy: Hereditary proximal spinal muscular atrophy (SMA) is characterized by lower motor neuron degeneration, progressive muscle weakness and atrophy and is caused by deficiency of the survival motor neuron (SMN) protein due to the homozygous loss-of-function of the SMN1 gene on chromosome 5q (Lefebvre et al., 1995, Wadman et al., 2018, Wijngaarde et al., 2020).