HBV infected serum has 3 types of viral particles by electron microscopy: ① intact particles: also known as Dane particles (HBV particles), with replication capacity and infectious. The outer shell protein HBsAg, the core contains HBV-DNA and HBVDNAp (DNA-polymerase), HBcAg, HBeAg; ② small spherical particles, tubular particles: no replication and infectious. The latter two are extra-viral (HBsAg) in excess of HBV replication and do not contain nucleic acids. HBV genome (HBV-DNA) consists of DNA with a double-stranded incomplete circular structure containing 3200 nucleotides. Due to its small host range, in vitro cell culture isolation of the virus has not been successful. In recent years, with the application of molecular cloning techniques and the successful transfection of in vitro cultured cell lines, the replication process of HBV has been further understood. HBV-DNA is divided into a negative strand (long strand) and a positive strand (short strand). The negative strand has four Open Reading Frames (ORF): ① S gene region, which consists of S gene, pre-S2 (pre-S2) gene and pre-S1 (pre-S1) gene. They encode HBsAg, pre-S, pre-S1 and poly-human serum albumin receptor (PHSA-R), respectively; (2) C gene region, which consists of pre-C gene and C gene. encoding HBeAg and HBcAg, respectively; (iii) the P gene region, encoding HBV-DNAp and having reverse transcriptase activity; (iv) the X gene region, encoding HBxAg and having the role of activating the HBcAg gene.