APOBEC4 is a putative cytidine deaminase of the AID/APOBEC family, whose physiological function remains incompletely characterized. Unlike other APOBEC family members with well-defined roles in innate and adaptive immunity, APOBEC4 appears to have distinct biological functions 1. The enzyme possesses a conserved zinc-coordinating deaminase motif typical of the APOBEC family 2, though it displays notably low mutagenic activity in standard assays 3. Emerging evidence suggests APOBEC4 plays a role in antiviral immunity: chicken APOBEC4 is highly expressed in lymphoid tissues and is upregulated during Newcastle Disease Virus infection, with ectopic expression reducing viral replication 2. In humans, APOBEC4 shows high expression in tissues targeted by SARS-CoV-2, suggesting potential involvement in coronavirus immunity 4. Notably, APOBEC4 demonstrates an unexpected pro-viral effect on HIV-1 replication, enhancing viral production in a dose-dependent manner without exhibiting detectable cytidine deamination activity, potentially through modulation of viral or host promoters 5. Additionally, APOBEC4 expression is transcriptionally regulated by MAF/MAFB and its dysregulation is associated with poor prognosis myeloma, linking it to aberrant mutagenesis in malignancy 6. These paradoxical findings indicate APOBEC4 possesses multifunctional roles distinct from canonical APOBEC-mediated antiviral restriction.