Expression of thisprotein is reduced in adults [98].However, the existence of the specific neuronal a-SMN isoform does not explainthe important fact of a lacking exon 7 in the SMN2 gene mRNAin most SMA cases, because only the first four exons in a-SMN are encoding ones[99]. The gene discussed is SMN1; the disease is proximal spinal muscular atrophy.