In collaboration with Alan McLachlan, Department of Microbiology and Immunology, Department of Medicine, College of Medicine at the University of Illinois at Chicago, we determined the role of hepatocyte nuclear factor 4alpha (HNF4alpha) in hepatitis B virus (HBV) transcription. Persistent viral infections requires the pathogen and host for extended periods of time without the host resolving the infection or the virus killing the host. Human immunodeficiency virus, herpes simplex viruses, papillomaviruses and hepatitis B and C virus have evolved a variety of strategies to persistently infect man. HBV chronically infects approximately 400 million people worldwide resulting in about a million deaths per year from liver cirrhosis and hepatocellular carcinoma. Most HBV infections worldwide occur in neonates at or around the time of birth and usually result in persistent infections. Although the immunological immaturity of the neonate is presumed to contribute to these chronic HBV infections, a precise understanding of the molecular events governing HBV persistence during development is lacking. HBV replicates its genomic DNA by reverse transcription of a pregenomic 3.5kb RNA which is transcribed from covalently closed circular 3.2kb viral genomic DNA in the nucleus of infected hepatocytes. Thus transcriptional regulation plays a central role in controlling viral replication levels and represents a potential antiviral target which has not, to date, been exploited clinically. In cell culture, the binding of HNF4alpha or retinoid X receptor alpha (RXRalpha) plus peroxisome proliferator-activated receptor &amp;#945;(PPARalpha) to the nucleocapsid promoter regulatory elements governs the level of synthesis of this critical HBV pregenomic 3.5kb RNA template. In this study, the level of HBV transcription and replication throughout early postnatal development correlates with the level of liver HNF4&amp;#945;expression in the HBV transgenic mouse model of chronic HBV infection was determined. The conditional depletion of HNF4alpha in the liver results in the loss of HBV transcription and replication indicating that this nuclear receptor is a major determinant of viral biosynthesis in vivo. These observations indicate that viral transcription, biosynthesis and antigen expression will increase progressively after infection at birth, possibly contributing to immunological tolerance and persistent infection. Additionally, the essential nature of HNF4alpha for host viability guarantees that HBV biosynthesis cannot be resolved without killing infected hepatocytes thus further limiting the hosts ability to resolve infection and increasing the probability of viral persistence.