C1D is a nuclear protein with critical roles in ribosomal RNA processing and transcriptional regulation. As a primary function, C1D recruits the RNA exosome complex to pre-rRNA substrates to mediate 3'-5' end processing of 5.8S rRNA 1. The protein contains a Sas10/C1D domain that forms a binding surface for protein-protein interactions while simultaneously engaging RNA or DNA substrates 2. Mechanistically, C1D associates with exosome subunit PM/Scl-100 in nucleoli and co-localizes with other exosome-associated proteins 1. C1D also functions as a transcriptional co-repressor/activator, regulating gene expression patterns that protect cells from oxidative stress 3. While C1D is not directly involved in nucleotide excision repair of UV-damaged DNA, it influences cell survival following UV irradiation through modulation of genes like DDIT3 and APEX1 3. Disease relevance includes genetic associations with antiphospholipid syndrome, where C1D variants reached genome-wide significance in genetic studies 4. Additionally, defects in C1D-related ciliary components cause primary ciliary dyskinesia with impaired mucociliary clearance 5. The protein's involvement in MAPK signaling regulation through protein degradation mechanisms suggests additional roles in cellular proliferation control 6. These multifaceted functions establish C1D as essential for maintaining genome stability, proper ribosome biogenesis, and cellular homeostasis.