HBV cccDNA in liver detected by nPCR. M: Molecular size standard, 100 bp ladder. 1: Negative control, sterile water. 2-5: Liver biopsies of tree shrews. Lanes 2, 3 and 5 showed positive bands at 356 bp. These three positive samples were collected from animal no. 90-1 at 2 years and 3 moths, no. 97-1 at 48 weeks and no. 98-2 at 42 weeks after inoculation, respectively. Sequence analysis showed they correspond to the sequence of HBV. 6: Positive control of HBV cccDNA, recombinant plasmid pUC18-HBV.