UBR5 is an E3 ubiquitin ligase that functions as a critical regulator of protein quality control and transcriptional networks across cytoplasmic and nuclear compartments. As a HECT-family ligase, UBR5 primarily acts as a ubiquitin chain elongator, extending pre-ubiquitinated substrates through branched Lys-11 and Lys-48-linked chains 1. In the cytoplasm, UBR5 partners with UBR4 to prevent protein aggregation by catalyzing heterotypic ubiquitin modifications on misfolded proteins, enabling p97/VCP-mediated segregation and proteasomal degradation 1. Nuclearly, UBR5 functions as an "orphan quality control" hub that recognizes and degrades unpaired transcription factors and ligand-bound nuclear receptors not associated with coactivators 23. This selective recognition of transiently exposed degrons maintains dynamic protein network composition while permitting responsive gene expression 3. Clinically, UBR5 dysregulation contributes to cancer pathogenesis. Elevated UBR5 in colorectal cancer promotes chemoresistance by stabilizing Smad3 via Lys-11 polyubiquitination, suppressing ferroptosis and oxaliplatin sensitivity 4. In ovarian cancer, tumor-derived UBR5 overexpression drives progression and metastasis through immunosuppressive macrophage recruitment while sustaining β-catenin signaling 5. Additionally, UBR5 mediates CDK1-driven oxaliplatin resistance by recruiting to phosphorylated ACSL4 for polyubiquitination, preventing ferroptosis-dependent cell death 6. These findings establish UBR5 as a potential therapeutic target for sensitizing chemoresistant cancers.