TUT1 (terminal uridylyl transferase 1) is a specialized RNA-modifying enzyme that functions primarily as a U6 snRNA-specific terminal uridylyltransferase essential for pre-mRNA splicing 1. The enzyme catalyzes the addition of oligouridine tails to the 3'-end of U6 snRNA, creating the Lsm-binding site required for U4/U6 di-snRNP formation and subsequent spliceosome assembly 12. TUT1 recognizes U6 snRNA through multiple domains including N-terminal zinc finger-RNA recognition motifs, catalytic domains, and a C-terminal RNA-binding domain, ensuring substrate specificity 13. The enzyme continues uridylylation until internal adenosine tracts in U6 snRNA can base-pair with the 3'-oligouridine tract 2. Beyond its role in splicing machinery assembly, TUT1 is critical for cancer cell survival, particularly in pancreatic adenocarcinoma, where it maintains tri-snRNP pools necessary for splicing transcripts with weak splice sites 4. TUT1 activity is regulated through ubiquitination by deubiquitinases USP15 and USP4, which control its subcellular localization between nucleolus and nucleoplasm 56. This regulation affects global RNA metabolism and maintains cellular homeostasis, with dysregulation linked to neurodegenerative processes and cancer progression.