An almost identical homologue, SMN2, which is always found in SMA patients, is unable to fully compensate for loss of SMN1 because of a translationally silent nucleotide change that alters the splicing pattern of the SMN2 gene, rendering most of its transcripts devoid of exon 7 [4], [5]. This evidence concerns the gene SMN1 and proximal spinal muscular atrophy.