SMN1 and proximal spinal muscular atrophy: SMA patients have homozygous loss or mutations of the SMN1 gene and retention of at least one copy of SMN2. Although the two SMN genes are nearly identical, a C to T transition in exon 7 of SMN2 disrupts splicing regulatory elements resulting mainly in the production of transcripts lacking exon 7 (SMNΔ7) with only a small proportion encoding full-length SMN [14]–[17].