SMN1 and proximal spinal muscular atrophy: When patients younger than 2 months were excluded due to variability in this age group, SMN protein levels were found to be associated with SMN2 copy number and were found to be significantly greater in SMA patients with 4 copies of SMN2, relative to patients with 2 and 3 copies of the gene (p < 0.0001) (Fig 3B).