SMN1 and proximal spinal muscular atrophy: Approximately 95% of SMA cases results from biallelic pathogenic sequence variations in the SMN1 gene, mainly represented by deletions including exon 7 of SMN1, or by the point mutation c.840C > T that converts exon 7 of SMN1 to SMN2, or by the presence of a hybrid SMN1-SMN2 gene.