Fialuridine, or 1-(2-deoxy-2-fluoro-1-D-arabinofuranosyl)-5-iodouracil (FIAU) is a thymidine nucleoside analog with activity against various herpesviruses and hepatitis B virus (HBV) in vitro and in vivo. Herpes virus thymidine kinase considered to be a target of FIAU. In a Phase II study, fialuridine induced a severe toxic reaction in chronic hepatitis B patients characterized by hepatic failure, lactic acidosis, pancreatitis, neuropathy, and myopathy. These clinical manifestations led to the hypothesis that the toxicity of FIAU was mediated through mitochondrial dysfunction, possibly as a result of the inhibition of mitochondrial DNA polymerase gamma and/or incorporation of FIAU into mitochondrial DNA.