APOBEC3H (A3H) is a single-stranded DNA cytidine deaminase that functions as a potent restriction factor against HIV-1 and other retroviruses 1. The protein exhibits significant genetic diversity, with seven haplotypes (I-VII) present in human populations, of which haplotype II shows the strongest antiviral activity 23. A3H restricts viral replication through both deaminase-dependent and -independent mechanisms. The deaminase-dependent pathway involves converting cytidine to uridine in single-stranded viral DNA during reverse transcription, leading to G-to-A hypermutations that impair viral fitness 1. Additionally, A3H can inhibit HIV-1 DNA synthesis through deaminase-independent mechanisms 1. The protein's antiviral activity is closely linked to its subcellular localization, with haplotype II being predominantly cytoplasmic while haplotype I localizes to the nucleus 3. A3H function requires RNA-mediated dimerization, and the protein remains bound to cellular RNA even during ssDNA interactions 4. Unlike other APOBEC3 proteins, A3H haplotype II is resistant to HIV-1 Vif-mediated degradation, making it a particularly effective restriction factor 25. The protein demonstrates processivity on single-stranded DNA through sliding, jumping, and intersegmental transfer mechanisms 6.