<h4>Objective</h4>To review the clinical characteristics and effect of treatment in patients with spinal muscular atrophy (SMA) and three copies of the <i>SMN2</i> gene.<h4>Methods</h4>We conducted a literature search in October 2022 to identify English-language clinical research on SMA that included <i>SMN2</i> copy number according to PRISMA guidelines.<h4>Results</h4>Our search identified 44 studies examining the impact of three <i>SMN2</i> copies on clinical characteristics (21 on phenotype, 13 on natural history, and 15 on functional status and other signs/symptoms). Here, SMN2 is linked to proximal spinal muscular atrophy.