RNF138 (ring finger protein 138) is an E3 ubiquitin ligase containing a RING finger domain, which mediates the ubiquitination and degradation of target proteins involved in diverse cellular processes. RNF138 is primarily localized in the nucleus and plays important roles in DNA damage repair, particularly in the homologous recombination (HR) pathway following double-strand DNA breaks. It regulates the recruitment and turnover of DNA repair factors through ubiquitin-dependent mechanisms, thereby contributing to genomic stability and cell survival under genotoxic stress. In addition to DNA repair, RNF138 has been implicated in the regulation of cell proliferation, cell cycle progression, and signal transduction. Dysregulation of RNF138 expression has been associated with multiple cancers, where it may function either as a tumor promoter or suppressor depending on cellular context.