BACKGROUND: APOBEC3H (A3H) is a member the Apolipoprotein B mRNA-editing catalytic (APOBEC) polypeptide seven cytidine deaminases family. These APOBEC3 enzymes have the innate ability to deaminate a cytidine base converting it to a uridine base. Previous research has suggested that human A3H haplotypes have similar properties like those of A3G and A3DE, in which these APOBEC3 enzymes are known to entail anti-viral activity toward viruses such as human immunodeficiency virus. METHODOLOGY: Human A3H gene was amplified by polymerase chain reaction and inserted into E.Coli plasmids. Expression tests were conducted to verify that the clones were able to express protein. Ten liters of the cloned cells were grown, and A3H protein was purified using an established protocol in Dr. Chen's lab for deamination to test the enzyme activity. An inactive mutant of A3H was also cloned to use as a negative control for the wild type A3H. PRINCIPAL FINDINGS: The isolated A3H haplotype 1 (referred to as A3H hereafter) is indeed active, but the activity is very weak in comparison to other APOBEC3 proteins, in which A3F and A3A were used as controls. A3H deamination is most optimal with an acidic environment of pH 5.5 or 6.0, and concentrated protein will produce more deamination product (darker bands). The inactive A3H produced some unexpected visible smaller DNA bands, which may be the result of contaminated protein activities. In conclusion, the wild type A3H protein may have weak activity, but additional studies are required to confirm that the inactivated A3H mutant has no activity on the DNA substrates.