However, in the literature, it has been more common to classify SMA by type rather than by copy number, and there is limited available data on populations exclusively composed of individuals with 4 copies of SMN2. Existing studies have reported significant variability in the clinical presentation and disease progression of SMA in individuals with 4 SMN2 copies (16, 17). This evidence concerns the gene SMN2 and proximal spinal muscular atrophy.