DNA deaminase (cytidine deaminase) which acts as an inhibitor of retrovirus replication and retrotransposon mobility via deaminase- dependent and -independent mechanisms (PubMed:16920826, PubMed:20062055, PubMed:21835787). Exhibits antiviral activity against HIV-1. 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:16920826). 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. Also inhibits the mobility of LTR and non-LTR retrotransposons (PubMed:27428332). {ECO:0000269|PubMed:12859895, ECO:0000269|PubMed:16920826, ECO:0000269|PubMed:20062055, ECO:0000269|PubMed:21835787, ECO:0000269|PubMed:22807680, ECO:0000269|PubMed:23097438, ECO:0000269|PubMed:23152537}. (Microbial infection) Enhances hepatitis B virus/HBV replication by excluding restriction factors APOBEC3F and APOBEC3G from HBV capsids. . This is the function of APOBEC3D (apolipoprotein B mRNA editing enzyme catalytic subunit 3D, Ensembl gene identifier ENSG00000243811).