Although the causative link between SMN deficiency and SMA was established 20 years ago4, it remains elusive how deficiency of a protein, which is ubiquitously expressed and causes widespread defects in pre-mRNA splicing in cell culture and mouse models of SMA, would result in a cell-type-specific phenotype: motor neuron dysfunction23. This evidence concerns the gene SMN1 and proximal spinal muscular atrophy.