SMN2 and proximal spinal muscular atrophy: Numerous studies have demonstrated an inverse relationship between SMN2 copy number and disease severity in SMA [13,37,49,50,54,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94].