DNA deaminase (cytidine deaminase) which acts as an inhibitor of retrovirus replication and retrotransposon mobility via deaminase-dependent and -independent mechanisms. Exhibits antiviral activity against viruse such as HIV-1 or HIV-2 (PubMed:15141007, PubMed:15152192, PubMed:23001005, 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. Selectively targets single-stranded DNA and does not deaminate double-stranded DNA or single- or double-stranded RNA. 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. May also play a role in the epigenetic regulation of gene expression through the process of active DNA demethylation. a 2'-deoxycytidine in single-stranded DNA + H(+) + H2O = a 2'-deoxyuridine in single-stranded DNA + NH4(+) Antiviral activity is neutralized by the HIV-1 virion infectivity factor (Vif), that prevents its incorporation into progeny virions by both inhibiting its translation and/or by inducing its ubiquitination and subsequent degradation by the 26S proteasome. Interacts with APOBEC3G in an RNA-dependent manner (PubMed:16699599). Interacts with AGO1, AGO2 and AGO3 (PubMed:22915799). (Microbial infection) Interacts with HIV-1 Vif (PubMed:15152192, PubMed:23001005). In the absence of Vif protein, specifically packaged into HIV-1 virions (PubMed:15152192). Widely expressed. Highly expressed in ovary. Up-regulated by IFN-alpha. The CMP/dCMP deaminase domain 1 mediates RNA binding, RNA-dependent oligomerization and virion incorporation whereas the CMP/dCMP deaminase domain 2 confers deoxycytidine deaminase activity and substrate sequence specificity. It is one of seven related genes or pseudogenes found in a cluster, thought to result from gene duplication, on chromosome 22. May be due to a competing donor splice site. Belongs to the cytidine and deoxycytidylate deaminase family.