SMN2 and proximal spinal muscular atrophy: Lapp et al. also noted that although a deficiency of SMN protein and transcripts is a pathological hallmark of SMA, their evaluation in body fluids appears to have limited utility due to weak correlations with clinical features, a limitation that may be more significant in adults with longer disease duration where compensatory mechanisms and chronic pathological changes could alter the relationship between SMN levels and functional outcomes [41].