SMN1 and proximal spinal muscular atrophy: Spinal muscular atrophy (SMA) is a neurodegenerative disorder of lower motor neurons from the spinal cord and motor nuclei of the brainstem, leading to severe proximal and symmetrical weakness associated with ventilatory insufficiency and spinal deformity.1The most prevalent form is caused by deletions or disease-causing variants in the survival motor neuron 1 (SMN1) gene, located in chromosome 5q (SMA-5q), which segregates as an autosomal recessive trait.2