UBL5 (ubiquitin-like 5) is an atypical ubiquitin-like protein with multifaceted cellular functions. Primary function: UBL5 plays a critical role in pre-mRNA splicing by associating with spliceosomal proteins and maintaining spliceosome integrity 1. It interacts with coilin to localize to Cajal bodies, nuclear domains where spliceosomal components assemble 2. Mechanism: UBL5 depletion decreases pre-mRNA splicing efficiency, leading to enhanced intron retention and subsequent defective sister chr19 cohesion through selective downregulation of the cohesion factor Sororin 1. UBL5 also promotes Fanconi anemia DNA repair pathway integrity by interacting with FANCI and mediating homodimer formation 3. Additionally, UBL5 provides protection against ER stress-induced apoptosis and inhibits viral infection by mediating degradation of viral proteins 4. Disease relevance: UBL5 variants associate with sleep duration regulation 5, and UBL5 downregulation occurs in heart failure patients, indicating involvement in mitochondrial quality control 6. Given its roles in DNA repair and splicing, UBL5 dysregulation may contribute to cancer development 7. Clinical significance: UBL5's evolutionary conservation across species and its essential role in genomic stability establish it as potentially important for cancer prevention and viral infection management.