We examined the effect of an anti-U1 ASO that sequesters the 5′ end of U1 snRNA on transcription and splicing of all internal exons of the spinal muscular atrophy (SMA) genes, SMN1 and SMN2. Our study was enabled by the employment of a multi-exon-skipping detection assay (MESDA) that discriminates against prematurely terminated transcripts. Here, SMN1 is linked to spinal muscular atrophy.