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AIM: To determine whether HBV with the same characteristics causes dissimilar mutations in different hosts.METHODS: Full-length HBV genome was amplified and linked with pMD T18 vector.Positive clones were selected by double-restriction endonuclease digestion (EcoR 1 and Hind Ⅲ) and PCR.Twenty seven clones were randomly selected from an asymptomatic mother [at two time points: 602 (1 d) and 6022 (6 mo)] and her son [602 (S)],and the phylogenetic and mutational analysis was performed using BioEditor,Clustal X and MEGA software.Potential immune epitopes were determined by the Stabilized Matrix Method (SMM),SMM-Align Method and Emini Surface Accessibility Prediction.RESULTS: All of the 27 sequences were genotype C,the divergence between the mother and son was 0%-0.8%.Compared with another 50 complete sequences of genotype C,the mother and her son each had 13 specific nucleotides that differed from the other genotype C isolates.AA 1-11 deletion in preS1 was the dominant mutation in the mother (14/18).The 1762T/1764A double mutation existed in all clones of the mother,3 of them were also coupled with G1896A mutation,but none were found in the son.17 bp deletion starting at nucleotide 2330 was the major mutation (5/9) in the son,which caused seven potential HLA class I epitopes and one B cell epitope deletion,and produced a presumptive new start codon,downstream from the original one of the P gene.CONCLUSION: The HBV strain in the son came from his mother,and discrepant mutation occurred in the mother and her son during infection.