The study investigates the role of U1 small nuclear RNA (snRNP) in regulating cancer cell migration and invasion. U1 snRNP, which silences proximal polyadenylation signals (PASs) in introns and exons, is inhibited by antisense morpholino oligonucleotides (U1 AMO). Low doses of U1 AMO (2.5–250 pmole) increase cancer cell migration and invasion by up to 500%, while higher doses are toxic. Conversely, U1 over-expression (U1 OE) reduces these phenotypes by 25–65%. High-throughput RNA sequencing reveals widespread transcriptome changes, including 3′ untranslated region (3′UTR) shortening, alternative splicing, and changes in mRNA expression levels of proto-oncogenes and tumor suppressors. These changes are consistent with U1's role in splicing and telescripting. The study suggests that U1 homeostasis, the balance between available U1 and transcription, plays a crucial role in maintaining normal gene expression and preventing the activation of oncogenic drivers and the downregulation of tumor suppressors. U1 is proposed as a potential target for modulating cancer cell behavior.The study investigates the role of U1 small nuclear RNA (snRNP) in regulating cancer cell migration and invasion. U1 snRNP, which silences proximal polyadenylation signals (PASs) in introns and exons, is inhibited by antisense morpholino oligonucleotides (U1 AMO). Low doses of U1 AMO (2.5–250 pmole) increase cancer cell migration and invasion by up to 500%, while higher doses are toxic. Conversely, U1 over-expression (U1 OE) reduces these phenotypes by 25–65%. High-throughput RNA sequencing reveals widespread transcriptome changes, including 3′ untranslated region (3′UTR) shortening, alternative splicing, and changes in mRNA expression levels of proto-oncogenes and tumor suppressors. These changes are consistent with U1's role in splicing and telescripting. The study suggests that U1 homeostasis, the balance between available U1 and transcription, plays a crucial role in maintaining normal gene expression and preventing the activation of oncogenic drivers and the downregulation of tumor suppressors. U1 is proposed as a potential target for modulating cancer cell behavior.