POLR1B encodes the catalytic core component of RNA polymerase I (Pol I), the DNA-dependent RNA polymerase responsible for synthesizing ribosomal RNA precursors. POLR1B participates in transcription of the 47S pre-rRNA precursor, which is processed into the 18S, 5.8S, and 28S ribosomal RNAs. The protein forms the active catalytic center with the largest Pol I subunit POLR1A, coordinating two magnesium ions to catalyze phosphodiester bond formation and facilitate Watson-Crick base pairing. POLR1B contributes proofreading activity through pausing and backtracking to remove misincorporated nucleotides. Pol I transcription initiation, elongation, and termination are essential for nucleolar structure and ribosome biogenesis; homozygous mutations in Polr1b cause preimplantation lethality and disrupt nucleolar organization through impaired phase separation in the granular compartment 1. Pathogenic variants in POLR1B cause Treacher Collins syndrome type 4 (TCS4), a rare autosomal dominant craniofacial disorder characterized by malar and mandibular hypoplasia 2. POLR1B variants induce p53-dependent apoptosis in the neuroepithelium, disrupting neural crest cell migration and differentiation 2. POLR1B is upregulated in non-small cell lung cancer and colorectal carcinoma, where it promotes cell proliferation and is associated with poor prognosis 3 4. These cancer associations suggest POLR1B as a potential therapeutic target, though no specific FDA-approved drugs targeting POLR1B are currently standard treatment.