Most cases of SMA are caused by homozygous loss of the survival of motor neuron 1 (SMN1) gene2. The survival of motor neuron 2 (SMN2) gene is a modifier of the SMA phenotype and has the nearly identical sequence to SMN1 with only a five-base pair difference3, 4. A C-to-T substitution in exon 7 of SMN2 leads to the skipping of exon 7 in approximately 90% of SMN2 transcripts5. The exon 7-skipped SMN2 protein cannot compensate for the SMN1 function because it is unstable and rapidly degraded. The remaining 10% of SMN2 transcripts is not sufficient to rescue the SMA phenotype. However, a higher copy number of SMN2 is associated with less severe clinical representations on average, even though it is not fully correlated1, 6.