RNGTT (RNA guanylyltransferase and 5'-phosphatase) is a bifunctional mRNA-capping enzyme catalyzing the first two steps of 5' cap formation on nascent pre-mRNA 1. The enzyme removes the gamma-phosphate from the 5'-triphosphate end of mRNA to generate a diphosphate intermediate, then transfers the GMP moiety of GTP to create the 7-methylguanosine cap structure 2. This co-transcriptional capping occurs during RNA polymerase II (RNAPII) transcription, with RNGTT docking to the RNAPII stalk and repositioning as RNA synthesis proceeds 23. The mRNA cap protects transcripts from degradation and recruits factors promoting splicing, export, and translation initiation 4. RNGTT expression is regulated by m6A methylation of its 3'UTR via METTL3, and reduced RNGTT expression impairs capping of ribosomal protein mRNAs, disrupting mTOR signaling 1. Beyond canonical capping, RNGTT regulates Hedgehog pathway signaling by antagonizing protein kinase A activity through cytoplasmic capping-enzyme functions 5. In liver regeneration, RNGTT controls biliary epithelial cell-to-hepatocyte transdifferentiation by transcriptionally regulating mTORC1 components and DNMT1 6. RNGTT functions within quality-control checkpoints at promoter-proximal pausing sites that remove defective RNAPII complexes 7, highlighting its essential role in transcriptional integrity and cellular proliferation.