SMN2 and proximal spinal muscular atrophy: The Δ7 model is a commonly-used model of severe SMA disease with a deletion of the mouse Smn gene, a complete human SMN2 transgene that predominantly generates an SMN2Δ7 transcript lacking exon 7, and multiple copies of the SMN2Δ7 cDNA, which results in a short-lived protein [36].