UBE4A is a U-box-type ubiquitin ligase that functions as both an E3 and E4 ligase, catalyzing Lys-48-linked polyubiquitination of substrates and facilitating polyubiquitin chain assembly. The protein is predominantly expressed in skeletal muscle, kidney, and liver, with nuclear and cytoplasmic localization. UBE4A regulates diverse cellular processes including proteasome biogenesis, metabolism, and antiviral immunity. It catalyzes ubiquitination of the transcription factor NRF1, promoting its DDI2-mediated cleavage and proteasomal gene expression 1. In metabolic homeostasis, UBE4A mediates K63-linked ubiquitination of Akt and APPL1 to facilitate insulin signaling; whole-body Ube4a knockout mice exhibit exacerbated obesity, hyperinsulinemia, and hepatic steatosis on high-fat diet 2. UBE4A also ubiquitinates apolipoprotein A-I for degradation, and blocking this pathway elevates circulating apoA-I to confer atheroprotection 3. In cancer immunity, UBE4A ubiquitinates PD-L1 for sorting onto tumor-derived extracellular vesicles, promoting anti-PD-1 therapy resistance 4. Biallelic loss-of-function variants in UBE4A cause syndromic intellectual disability with developmental delay, hypotonia, seizures, and behavioral abnormalities, indicating essential roles in neurological development 5. In antiviral defense, UBE4A-mediated K6-linked ubiquitination of Viperin targets it for degradation; UBE4A inhibition restores Viperin levels and enhances antiviral responses 6. UBE4A represents a potential therapeutic target in cancer immunotherapy, metabolic disease, and antiviral treatment.