We produced transgenic mice by microhijecting a partial tandem duplication of the complete hepatitis B virus (HBV) genome into fertilized eggs of C57BL/6 mice. One of eight transgenic mice was a high producer for HBV surface antigen (HBsAg) and HBV e antigen (HBeAg) in the serum. The HBV genomes were transmitted to the next generation and these F1 mice also produced HBsAg and HBeAg. mRNAs of3.5, 2.1, and 0.8 kilobases were detected in the livers and the kidneys of these mice. In addition, a 0.8-kilobase RNA was detected in the testis. Single-standed and partially doublestranded HBV DNAs were shown to be produced in the cytoplasm of the liver and kidneys. These HBV DNAs were associated with the core particles, indi ble from nudeocapsid produced in an infected human liver. Viral genome DNA was detected in the serum. These results demonstrate that the HBV genome integrated into the mouse chromosome acted as a template for viral gene expression, allowing viral repliction. Thus, these tranic mice should be useful for detailed studies of the replication and expression of HBV and for pathological studies of hepatitis, including the development of hepatocellular carcinoma.Hepatitis B virus (HBV) is a causative agent of hepatitis. This viral genome DNA is a partially double-stranded circular molecule. After infection, it is converted into a covalently closed circular molecule (1), which is transcribed into two main species of mRNA, 2.1 and 3.5 kilobases (kb) in size (2). These molecules are then translated to produce viral proteins, HBV surface antigen (HBsAg) and HBV core antigen (HBcAg), and presumably other proteins called Pre-S, X, and Pol as inferred from the open reading frames. The 3.5-kb RNA, which is called pregenome RNA, is reverse transcribed (3) presumably by the viral polymerase (4), and the product, single-stranded minus DNA, then serves as a template for the synthesis of a plus strand. This reaction often terminates before completion, resulting in the formation of partially double-stranded DNA.HBV infection is linked to later development ofcirrhosis and hepatocellular carcinoma (HCC). Beasley etaL (5) showed from prospective epidemiological studies that the relative risk to HBsAg carriers ofdeveloping HCC was 217 times as compared with noncarriers. Despite the crucial role of HBV in human health problems, there is only limited knowledge of its mode of replication, integration, and tumor induction because the virus multiplies only in human and chimpanzee livers. Recently, cell culture systems have been established that allow expression and replication of the HBV genome following transfection with cloned HBV DNA (6-9). These systems have allowed detailed molecular and genetic studies of HBV replication and protein synthesis, but they are not suitable for studies on the outbreak of hepatitis and the induction of HCC. One approach to overcoming these problems is to make a transgenic animal carrying HBV DNA. In this approach the introduced DNAs are located on the same chromosomal site in all cell types ...