SMN1 and proximal spinal muscular atrophy: SMA exhibits locus heterogeneity; however, the vast majority of SMA patients (~96%) segregate biallelic loss-of-function variants in the survival motor neuron 1 gene (SMN1), a gene that encodes a protein involved in small nuclear ribonucleoprotein biogenesis (snRNP) and pre-mRNA processing [4, 5].