SMN1 and proximal spinal muscular atrophy: Interestingly, they further observed that SMN protein levels across spMN with the same genetic background (derived from healthy individuals or from patients affected by different SMA severities or fALS forms) were highly heterogeneous [157] and that such heterogeneity directly correlated with the spMN probability of survival, with the low-SMN expressing MNs being more vulnerable to different stressors.