SMN1 and proximal spinal muscular atrophy: Notably, an ASO approach tested extensively in genetically humanised transgenic SMN2 mice (a gene not present in mouse) blocks an intronic splicing silencer in human SMN2, which increases full-length SMN2 isoform expression and compensates for loss of SMN1 that causes SMA (Hua et al. 2011; Passini et al. 2011; Porensky et al. 2012; Williams et al. 2009b).