RNF166 (ring finger protein 166) is an E3 ubiquitin ligase that regulates diverse cellular processes through substrate-specific ubiquitination mechanisms. The protein mediates different types of polyubiquitin linkages, including K29- and K33-linked ubiquitination of SQSTM1/p62 during antibacterial autophagy, promoting xenophagic targeting of bacteria 1. RNF166 also catalyzes K63-linked ubiquitination, as demonstrated with Ku80, leading to increased protein stability 2. Beyond ubiquitination, RNF166 exhibits dual functionality by inducing sumoylation of targets like ASK1 2. The protein contains a unique Di19-ZF domain that recognizes poly(ADP-ribose) modifications, enabling it to function as a reader-writer E3 ligase that extends K11 polyubiquitin onto MARUbylated proteins 3. Pathologically, RNF166 promotes apoptosis in neurodegeneration by ubiquitinating and degrading XIAP 4, contributes to diabetic nephropathy through mitochondrial dysfunction via CYLD regulation 5, and facilitates colorectal cancer progression by destabilizing poly-ADP-ribosylated angiomotins 6. RNF166 has also been identified as a potential biomarker in atrial fibrillation-related stroke 7, highlighting its clinical relevance across multiple disease contexts.