RNF8 is an E3 ubiquitin ligase that serves as a central hub in DNA damage signaling, orchestrating both histone ubiquitination and protein degradation through distinct lysine-linkage mechanisms 1. Upon double-strand breaks (DSBs), RNF8 catalyzes Lys-63-linked ubiquitination of histones H2A and H2AX, promoting recruitment of repair proteins TP53BP1 and BRCA1 to damage sites 2. Simultaneously, RNF8 mediates Lys-48-linked ubiquitination to degrade target proteins like KU80 and JMJD2A, facilitating non-homologous end joining 3. RNF8's role extends beyond canonical DSB repair: it ubiquitylates XRN2 to resolve R-loops, and its loss in BRCA1-mutant cells causes synthetic lethality through R-loop accumulation 4. Additionally, RNF8 functions at uncapped telomeres by promoting histone H2A ubiquitination and TP53BP1 recruitment, though this may paradoxically enhance genome instability during telomeric crisis 5. In spermatogenesis, RNF8 mediates histone-to-protamine exchange, with MIWI sequestration of RNF8 causing male infertility 6. RNF8 is also critical for optimal cell cycle checkpoint activation; UBE2T-mediated H2AX monoubiquitination by RNF8 facilitates CHK1 activation, regulating radiotherapy sensitivity 7. These multifaceted functions implicate RNF8 dysregulation in cancer development and therapeutic resistance.