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Hepatitis B virus (HBV) is characterized with high mutations,which is attributed to the lack of proof-reading of the viral reverse transcriptase and host immune pressure.In this study,31 HBV chronic carriers from 14 families were enrolled to investigate the evolution of the same original HBV sources in different hosts.Sequences of pre-C and C (pre-C/C) genes were analyzed in eight pairs of HBV-infected mothers with longitudinal sera (at an interval of 6.0-7.2 years) and their children (5.5-6.7 years old),and in 15 adults (21-78 years old) from six families with known intrafamilial HBV infection.The pre-C/C sequences had almost no change in eight mothers during 6.0-7.2 years and their children who were in immune tolerant phase.The pre-C/C sequences from the 15 adults of six families,mostly in the immune-clearance phase or the low replicative phase,showed various diversified mutations between individuals from each family.Compared to a reference stain (GQ205441) isolated nearby,the pre-C/C in individuals in immune tolerant phase showed 98.56％-99.52％homology at nucleotide level and 99.5％-100％ homology at amino acid level.In contrast,multiple mutations were developed in the immune-clearance phase or the low replicative phase,affecting immune epitopes in core gene and G1896 in pre-C gene.The results indicate that the evolution of new HBV variants is not mainly resulted from the spontaneous error rate of viral reverse transcription,but from the host immune pressure.