POLR2D (RNA polymerase II subunit D) encodes RPB4, a core component of RNA polymerase II essential for mRNA synthesis and transcription regulation 1. As part of the dissociable RPB4-RPB7 subcomplex, POLR2D helps lock the RNA polymerase II clamp in closed conformation and binds single-stranded DNA and RNA 1. The protein is essential for vertebrate development, as zebrafish polr2d mutants exhibit delayed somitogenesis, increased cell death, and embryonic lethality by 60 hours post-fertilization 1. POLR2D expression is diminished in mutants, confirming its critical role in housekeeping and zygotic gene expression 1. The gene is subject to complex post-transcriptional regulation through miRNA-mediated silencing, particularly by let-7b, which is facilitated by RNA-binding proteins like AUF1 2. This regulatory mechanism can be exploited by viral proteins, as SARS-CoV-2 Nsp9 suppresses POLR2D expression by promoting let-7b-mediated silencing 3. Clinically, POLR2D is commonly overexpressed in multiple cancer types and serves as a prognostic biomarker, with overexpression associated with decreased survival in glioblastoma and colon adenocarcinoma 456. The protein's role extends beyond basic transcription to specialized functions in cancer cell proliferation 4.