The most common form of SMA is caused by biallelic pathogenic variants in SMN1. The vast majority of SMA molecular diagnoses (~95%) are due to homozygous deletion of the entire SMN1 gene, while a minority (~5%) are due to a compound heterozygous sequence variant in SMN1 on one chromosome and an SMN1 gene deletion on the other chromosome (4). Here, SMN1 is linked to proximal spinal muscular atrophy.