In addition to the SMN2 copy number, there are genetic modifiers found in asymptomatic SMN1-deleted individuals in SMA discordant families, such as Plastin 3 (PLS3, [MIM: 300131]) and Neurocalcin delta (NCALD, [MIM: 606722]) that influence the severity of SMA [13, 14]. Here, SMN2 is linked to proximal spinal muscular atrophy.