SMA is caused by homozygous deletions in the survival motor neuron gene 1 (SMN1), which significantly reduces SMN protein levels and leads to SMA-specific changes in motor neurons, including hyper-activation of the endoplasmic reticulum (ER) stress pathway [6], resulting in the loss of lower α-motor neurons in the spinal cord [7,8]. Here, SMN1 is linked to proximal spinal muscular atrophy.