Recently, the axonal-SMN (a-SMN) protein, which is generated by the gene responsible for spinal muscular atrophy (SMA), SMN, has been reported. Surprisingly, the a-SMN transcript includes the entire sequence of SMN intron 3. We had expected a high frequency of insertion/deletion mutations at a polyadenine tract in this intron, since simple repetitive sequence motifs are prone to mutations. Such mutations could change the C-terminal structure of the a-SMN protein. However, our study showed that almost all individuals, including healthy individuals, SMA patients and SMA-like patients, carried only alleles with a normal polyadenine tract. Hypomutability of the polyadenine tract in SMN intron 3 suggests the existence of transcriptional mechanisms preventing alterations to the open reading frame of axonal SMN and not allowing variability in the protein structure of a-SMN.