In the absence of the human immunodeficiency virus type 1 (HIV-1) Vif protein, the host-cell cytidine deaminases APOBEC3F and -3G are co-packaged along with virion RNA. Upon infection of target cells, nascent single-stranded DNA can be edited extensively, invariably giving rise to defective genomes called G-->A hypermutants. Although human T-cell leukemia virus type 1 (HTLV-1) replicates in the same cell type as HIV-1, it was shown here that HTLV-1 is relatively resistant to the antiviral effects mediated by human APOBEC3B, -3C, -3F and -3G. Nonetheless, a small percentage of genomes (0.1<f<5 %) were edited extensively: up to 97 % of cytidine targets were deaminated. In contrast, hypermutated HTLV-1 genomes were not identified in peripheral blood mononuclear cell DNA from ten patients with non-malignant HTLV-1 infection. Thus, although HTLV-1 DNA can indeed be edited by at least four APOBEC3 cytidine deaminases in vitro, they are conspicuously absent in vivo.