RTCB (RNA 2',3'-cyclic phosphate and 5'-OH ligase) is a 3'-5' RNA ligase and catalytic subunit of the tRNA-splicing ligase complex that catalyzes phosphodiester bond formation between RNA fragments 1. In canonical tRNA splicing, RTCB functions downstream of tRNA endonucleases, ligating tRNA exons through a three-step GTP- and Mn(2+)-dependent mechanism requiring Archease-mediated active site guanylylation 1. RTCB also performs non-canonical splicing of XBP1 mRNA during endoplasmic reticulum stress, where IRE1/ERN1-generated exon ends are ligated to produce the active XBP1s transcription factor 2. Beyond these classical roles, RTCB participates in emerging biological processes: it ligates mRNA fragments in SOS splicing, a spliceosome-independent system protecting genes from DNA transposon-mediated disruption 3, and it enables in-cell circularization of exogenous RNAs for improved persistence in cell engineering applications 4. Dysregulation of RTCB has clinical significance in cancer, where the USP45-RTCB-DDX1 axis promotes tumorigenesis and chemoresistance through post-translational stabilization mechanisms, with elevated expression correlating with poor patient survival 5. These diverse functions establish RTCB as a fundamental regulator of RNA metabolism with implications for protein synthesis, cellular stress responses, and disease pathogenesis.