The survival of motor neuron (SMN) genes, SMN1 and SMN2, are closely related to the development and severity of spinal muscular atrophy (SMA).1 Mutations in SMN1, but not in SMN2, are recognized as causing SMA because SMN1 is completely deleted in more than 95% of SMA patients and is deleteriously mutated in the remaining patients.2SMN2 is now considered to be a modifying factor of the SMA phenotype.3,4 Without exception, all SMA patients retain SMN2.2 This evidence concerns the gene SMN2 and proximal spinal muscular atrophy.