SMN2 and proximal spinal muscular atrophy: Studies in more accessible tissues suggest that SMN levels may decline further throughout life in patients with SMA (Ramos et al., 2019; Wadman et al., 2016) or remain relatively stable – especially in adults – depending on the cell type studied (Groen et al., 2018b; Kobayashi et al., 2011; Zaworski et al., 2016).