DNA deaminase (cytidine deaminase) with restriction activity against viruses, foreign DNA and mobility of retrotransposons. Exhibits antiviral activity against adeno-associated virus (AAV) and human T- cell leukemia virus type 1 (HTLV-1) and may inhibit the mobility of LTR and non-LTR retrotransposons. Selectively targets single-stranded DNA and can deaminate both methylcytosine and cytosine in foreign DNA. Can induce somatic hypermutation in the nuclear and mitochondrial DNA. May also play a role in the epigenetic regulation of gene expression through the process of active DNA demethylation. {ECO:0000269|PubMed:10469298, ECO:0000269|PubMed:12859895, ECO:0000269|PubMed:16527742, ECO:0000269|PubMed:19461882, ECO:0000269|PubMed:20062055, ECO:0000269|PubMed:20615867, ECO:0000269|PubMed:21123384, ECO:0000269|PubMed:21368204, ECO:0000269|PubMed:21460793, ECO:0000269|PubMed:21496894, ECO:0000269|PubMed:22457529, ECO:0000269|PubMed:22896697}. This is the function of ENSG00000128383 (APOBEC3A, apolipoprotein B mRNA editing enzyme catalytic subunit 3A).