EXO1 and hepatocellular carcinoma: To investigate the effects of the WHc mutations on cccDNA formation, we extracted extrachromosomal DNA (i.e., protein-free DNA or PF-DNA, including viral DNA without the attached RT protein as well as plasmids) from the WHV replicon-transfected human and woodchuck hepatoma cells by Hirt extraction and detected cccDNA after Dpn I digestion followed by exonuclease I and III (Exo I/III) treatment, which removes plasmid DNA as well as non-covalently closed viral and cellular DNA [37,51].