DNA deaminase (cytidine deaminase) which acts as an inhibitor of retrovirus replication and retrotransposon mobility via deaminase- dependent and -independent mechanisms (PubMed:15141007, PubMed:20062055, PubMed:22915799, PubMed:34774569). Exhibits antiviral activity against viruse such as HIV-1 or HIV-2 (PubMed:15141007, PubMed:15152192, PubMed:20219927, PubMed:21835787, PubMed:22807680, PubMed:23001005, PubMed:23097438, PubMed:23152537, PubMed:34774569). After the penetration of retroviral nucleocapsids into target cells of infection and the initiation of reverse transcription, it can induce the conversion of cytosine to uracil in the minus-sense single-strand viral DNA, leading to G-to-A hypermutations in the subsequent plus- strand viral DNA (PubMed:15141007). The resultant detrimental levels of mutations in the proviral genome, along with a deamination-independent mechanism that works prior to the proviral integration, together exert efficient antiretroviral effects in infected target cells (PubMed:15141007). Selectively targets single-stranded DNA and does not deaminate double-stranded DNA or single- or double-stranded RNA (PubMed:15141007). Exhibits antiviral activity also against hepatitis B virus (HBV), equine infectious anemia virus (EIAV), xenotropic MuLV- related virus (XMRV) and simian foamy virus (SFV) and may inhibit the mobility of LTR and non-LTR retrotransposons (PubMed:16378963, PubMed:16527742, PubMed:19458006, PubMed:20062055, PubMed:20335265). May also play a role in the epigenetic regulation of gene expression through the process of active DNA demethylation (PubMed:21496894). {ECO:0000269|PubMed:15141007, ECO:0000269|PubMed:15152192, ECO:0000269|PubMed:16378963, ECO:0000269|PubMed:16527742, ECO:0000269|PubMed:19458006, ECO:0000269|PubMed:20062055, ECO:0000269|PubMed:20219927, ECO:0000269|PubMed:20335265, ECO:0000269|PubMed:21496894, ECO:0000269|PubMed:21835787, ECO:0000269|PubMed:22807680, ECO:0000269|PubMed:22915799, ECO:0000269|PubMed:23001005, ECO:0000269|PubMed:23097438, ECO:0000269|PubMed:23152537, ECO:0000269|PubMed:34774569}. This is the function of APOBEC3F (apolipoprotein B mRNA editing enzyme catalytic subunit 3F, ENSG00000128394).