DNA deaminase (cytidine deaminase) which acts as an inhibitor of retrovirus replication and retrotransposon mobility via deaminase-dependent and -independent mechanisms. Selectively targets single-stranded DNA and does not deaminate double-stranded DNA or single- or double-stranded RNA (By similarity). Exhibits single-stranded DNA deaminase activity (in vitro) (PubMed:26491161). Incorporates into the released virions of the virion infectivity factor (vif)-deficient feline immunodeficiency virus (FIV) and suppresses FIV infectivity, probably in a deaminase-dependent manner (in vitro) (PubMed:26491161). Induces G-to-A hypermutations in vif-deficient FIV (in vitro) (PubMed:26491161). The APOBEC3H/APOBEC3Z3 haplotype 5 exhibits antiviral activity against vif-proficient FIV, strains Petaluma, C36 and Shizuoka (in vitro) (PubMed:26491161). Does not exhibit inhibitory activity against feline leukemia virus (FeLV), feline endogenous retrovirus (RD-114 virus) or a long interspersed nuclear element-1 (LINE-1) retrotransposon (in vitro) (PubMed:26491161). a 2'-deoxycytidine in single-stranded DNA + H(+) + H2O = a 2'-deoxyuridine in single-stranded DNA + NH4(+) Expressed in peripheral blood mononuclear cells. There are at least 7 different APOBEC3H/APOBEC3Z3 haplotypes in the feline population. The displayed haplotype 1 is degraded in a feline immunodeficiency virus (FIV) virion infectivity factor (vif)-dependent manner and is inefficient to block vif-proficient FIV replication. Haplotype 5 is resistant to vif-mediated degradation and is able to suppress vif-proficient FIV infectivity. Belongs to the cytidine and deoxycytidylate deaminase family.